Sympathy for the Devil; Elle2's thoughts:
AC/DC is always a good opener, not a song I'm that familiar with so I couldn't get all the words, but it did its job very well, Season 4 was recapped breathlessly and then we were launched into Season 5 - what a setup!
In typical Eric Kripke fashion this script contained a lot, so much that repeated viewings are necessary - poor us. There is a lot to love about this script and some moments where I said "yeah, but what about" and while there were some pacing issues, on the whole it is better than its parts. Its no "Devil's Trap" or "IMTOD" or "NRFTW" or "Lazarus Rising" but its no "Magnificent Seven" or "All Hell Breaks Loose, II." I've watched it three times and I like it better each time and as a setup for the entire season to come, it does its job excellently.
My opinion is that Mark Pellegrino is perfectly cast as Lucifer. I remember the casting call specifically stated that someone with 'piercing eyes' or something to that affect is what they were looking for. Mark's eyes, to me, have a quality that will be put to great use in this role; I'm looking forward to this character. As for the SPN spin on Lucifer, we see him devious in how he coaxes his 'host' into saying yes; playing on his grief, perhaps guilt that he survived - even so much as covering his hands with what is presumably meant to be his wife's blood, ergo her blood is on his hands - tortures him with the memories and the pain and then offers what any grief-stricken individual is likely to want, someone to blame and a chance to 'get even.' 
Do I have sympathy for the host of Lucifer, you betcha; do I have sympathy for the devil, nope. Regardless of SPN's spin, "I loved God too much" or the reality which is Lucifer loved himself too much, Lucifer fell because of choices he made and a belief that he was somehow greater than God; pride will get you every time.
Chuck's alive!!! Yeah, not that much of a surprise as I've read the interviews where we were told that Chuck would be back. I like the character and like that the boys have someone that helps them out. I firmly believe that while Kripke and Co. like what they have developed with the whole 'Prophet Chuck' they are not going to fall into any Roadhouse trap. I'm betting on snippets of Chuck when appropriate but not so much that we get tired of him - besides, he has a lot of cleaning to do.
Ah, in true Kripke fashion he has great fun with the fandom - slash fic sickens me intensely so I found the fangirl awful to sit through the first time, not that she did a bad job, she was just over the top without going too far over the top and I enjoyed it better the second time especially Sam's intense discomfort. Apparently we're likely to play off of Jared's physique again this season and that's fun all in itself as Sam is the less forward of the brothers so it's fun to watch him squirm; even more fun to watch Dean in the background as he struggles to understand the attraction and perhaps enjoy a bit of his brother's discomfort.
Finally, Castiel kicks serious butt! I've been waiting for Castiel to have a really good fight scene and he finally got it, way to go. His arrival wraps two good mysteries together and leaves us hanging as for the answer; how and why did the boys get on that 'Soul Plane' and how did Castiel "come back to life." Those are two hanging questions that Kripke has posed and will no doubt unravel as the season progresses; I'm betting on God - not a big stretch for Castiel nearly spelled it out for Zachariah.
Bobby, you remain the awesomeness of awesome; you'll sacrifice yourself to save Dean - hmm, that sounds familiar. I don't know how the knife to the abdomen killed the demon without killing the host or how that translates to Bobby never walking again - did he pierce so far as to hit his spinal cord? Sure, I know Zach said he'd make it so that Bobby never walked again and Castiel only told him to put the boys back together again - great line, BTW - but still, how does that come to be? I don't know and I feel real bad for Bobby that we're left with the idea that the boys left him in that hospital but perhaps they're just heading to the local motel for the night. Bobby's got his own story arc for Season 5 and that's good with me.
I'm guessing that the reason we saw the Impala arrive and the legs of 'Bobby' walk in front of that fabulous grille is because they needed a filler scene but Jim Beaver was no longer on set so any set of legs would do - I'll offer mine, I like boots and jeans - but that's just me liking to guess a little now and again as to why a shot is the way it is. 
I do have to question the timing and/or sequence of Sam and Dean at Chuck's. The way that scene unfolded it appears to me that the boys are exploring the house upon arrival and then they find Chuck and then the angels appear, when did Dean cut his hand to make the angel banishing symbol, and why later was he not wearing a bandage on his hand? Again, minor, minor nitpicks but I pick at a nit every now and again so there you go - hmm, that doesn't sound very good. *blushes*
How lucky were we, many of us said we wanted to see John's storage place again and boom, there it was. Thank you, Mr. Kripke. I enjoyed looking around a bit more - what do they do with all the bodies? Hope the boys remember to disable the cameras as they shuffle those bodies out of there - they have a lot on their minds, so they might forget.
Sam's blood is cleaned? I don't mind that, at least the part where he doesn't have the huge withdrawal like WTLB, really, we've been there so to do it again would be redundant. I firmly believe though that while Sam may be over the withdrawal aspects of the demon blood he will forever be tempted by it and that is not easy to live with. He's got a tough road ahead of him, not only does Dean not trust him, he's likely to question every move he makes, not good when hunting. Sam remains committed to the fight, however, he also seems willing to follow Dean's lead - hmm, we haven't seen that in a while. As they left the hospital Sam was offering some suggestions on how to combat the devil, as in he appeared to believe Dean really had a plan. I like that Sam is listening to Dean; too bad he was in for a huge fall.
Dean is Michael's vessel? Wow, did not see that coming. I like it though for true to Dean's character, he refuses destiny and declares his free will. What's fresh for Dean is that when confronted with Bobby's disability or Sam's death he did not give in. No surprise he was willing to suffer himself, he's smart enough to know that they'll not kill him, they need him for Michael also, since he's sacrificed himself before, it's not that hard of a stretch to figure he'd be willing to do it again, but this time he refuses to heal Bobby and refuses to save Sam; Dean's coming to be his own person, finally. Dean's fatalism remains firmly in place, though. "I'll fight, but let's be at least realistic, we don't have a snowball's chance and you know it." That's right, Dean, way to be real. I'm glad those lines were there though along with Dean's comment about saying all that stuff for Bobby's benefit; the scene with Dean's big speech at the hospital was a bit over the top, however I judge the episode as a whole and since that was rapidly followed by his revelation that he only did that for Bobby's sake makes it very palatable.
Meg returns, sort of. I mostly liked this Meg, mostly. She got a little silly with the whole, super demon powers to Sam but overall I liked this new incarnation. There was enough of Nikki's Meg in Rachel's Meg for me to enjoy her appearance; I figure we'll see her again.
So what did Eric hint at for the rest of the season besides Bobby's arc dealing with his infirmities, Dean coming to grips with being Michael's vessel, Sam struggling with his time in his own personal hell and learning to trust himself again, Castiel dealing with obviously being saved by God? Hmm, we heard the Colt mentioned, first time in a long time, we saw John's storage unit so perhaps there will be something more coming from there, we know the brothers will have to work very hard at mending their relationship, they'll need to avoid any x-rays of their ribcages at any hospitals - hard to explain a sigil carved into your ribs - Dean will struggle with being a leader, chosen by fate much like Sam struggled with being a leader, chosen by fate; once again the brothers' journeys mirror each other, I like that. 
"Sympathy for the Devil" is a solid season opener, it offers some answers, or at least alludes to some, it poses new questions and plot directions, it had humor, action, goofiness, the return of old enemies and the arrival of a new and it made it clear that while the brothers are together they are also very much apart. I guess I do have sympathy for the devil after all; he's facing two Winchester brothers with a lot to prove to each other, themselves and to Heaven and Hell, bring it!
Thanks for reading.