Where to start...so many things to talk about.
I personally loved this episode- great dialogue and great story evolution, which is nothing less than what I would expect from Ben Edlund. Up until now this season had been very slow developing but this seems to have moved it up notch (as would be expected 3 episodes before the season finale). Cas's "I don't like conflict" and "pull my finger" comments, and his attempts to still try and be relevant to his former colleagues were hilarious and a pleasure to watch. Likewise, the angels reactions to his return were quite interesting. Hestia being quite righteous and angry at Castiel's disappearance and his introduction of free will, and the other angel's comment about there being so few of them left, gives us a good indication of how things have been up in heaven since we last saw it.
Furthermore, while everyone is entitled to his or her opinion, I have to disagree that Cas doesn't seem like Cas. Save for the fact that he swallowed Sam's insanity, and that it is now understandably manifesting itself in a different form, he is still very much Cas- awkward, caring, comical and a lynchpin for the brothers. He's been around for just about 4 seasons now and, as such, I would argue that some character developments, even if they are minor or temporary, are pretty much expected.
When it comes to Meg, while I agree that it is highly weird seeing her help Cas, and even weirder that Cas would accept such help, when one thinks about it, it all does sort of make sense. First off, as many people have noted, Meg is self serving so if having an angel in her pocket serves her, of course she's going to help Cas (come to think of it, since Meg is going against Crowley, I can't help but wonder if Meg and Dick might be working together). More importantly, given Cas's history of rebellion, it's not that crazy that he would form a relationship with Meg. In fact, if any angel would, it would be him. It's just in his nature to do things other angels wouldn't.
As far as the angel's being killed by Leviathan thing goes, while I do think there are those who have made some good points about it being weird, I'm going to go ahead and defend the end outcome. If rules had already been established about the powers of the Leviathan (E.G- Angels being able to beat them) what happened would be dues ex machina. However, because so little was known about them before, I don't have a problem with them being able to kill angels.
To me, it's a lot like the movie Signs. A lot of people complained that it was stupid to create a species that would invade a planet that was made up with over 3/4s of something they were allergic to. However, if they had never encountered water before, how would they have known they were allergic to it? Same thing goes for the angels and Leviathans here. If angels had never encountered Leviathan before, how would they know how to fight them or recognize them? Furthermore, if anyone has ever been in fight, or a life or death situation, they probably know there are usually three responses one has- fight, flight, or freeze. Yes, after his first attack failed the angel could have just flung a table or a microwave at Edger's head, but he was understandably scared and realized he was probably about to die. Hence, with only a second or two to react, it's not all that surprising that the he froze. In a fair fight I think it would be much more even matched, but this was very much a "by surprise" attack orchestrated by a top level leviathan against two lower level angels.
Lastly, I'm glad we finally got to see all three of the shows major supernatural beings in one episode- angels (Heaven), leviathans (Purgatory) and demons (Hell). It was about time and hopefully we'll get to see a bit of a three way stand off between all them at some point in the next few episode. Edger's comment about "paper, rock, scissors- leviathan beats angel", and Crowley's ability to teleport away from Dick in a previous episode makes me wonder if demons might have more of an edge against the leviathan than even Crowley himself seems to know (given that neither angels or demons seem to know much about them).
Anyways, definitely a great episode and looking forward to the last two.