Naturally I saw this already.
For what it is worth, I would still put both Nolan-Batman films ahead of it. I think this is roughly where it will end up on imdb/metacritic type scores too (it's ahead of Batman Begins but well behind Dark Knight right now, and will probably be well behind the third one too later in the summer). For my personal taste, I tend to like darker and rougher territories to be covered in my favourite media. Hence, I like the Wire or Game of Thrones as shows as opposed to sitcoms or even other dramas, and I will thus prefer Batman, in its modern form, to Iron Man. That's not to say that there are not some flaws in the Batman films that aren't necessarily in the Avengers, but it's also almost an entirely different genre of film to the typical superhero action setup with very different questions and answers to problems.
While it's been pointed out to me that as a guy I should like movies where things blow up, and I don't deny that I enjoy some such films where there is a good deal of action and explosions, I tend to prefer they be accompanied by other things. Like a good use of humor, a clever plot of some sort, a decent villain or a sensible basis for things to be exploding, coherent fights, and so on. Avengers had most of these features, as did the first Iron Man where many of its prequel portions did not (the first Hulk movie in particular, the second Iron Man, Captain America, and Thor were all weak in some of these regards too). I therefore liked it and would probably go see it again. I had a hard time following some of the fights from where I had to sit given how crowded the theater was. Most of them were quite good at living up to expectation however (Thor v Hulk, Thor v Iron Man, Loki v Cap, Hulk v Loki, both Hawkeye and Black Widow have some serious moves too.) I also appreciated BW getting some roles in the plot rather than just being eye candy as is typical of teen-action flicks and any attractive women being mostly if not completely useless (also a reason I look forward to seeing Catwoman in the next Batman movie). I credit Whedon for that and for the overall banter and humor that was pulled off.
Other notes: I'm not sure if they filmed the whole thing in 3D or just converted it, but my suspicion is on the latter. It was rare that I noticed the 3D features (a couple shots of Iron Man changing directions or fighting things in mid-air is about all). It's probably not worth that much to see it that way. That last Transformers movie was much more obvious where the money spent on 3D went and was thus more visually interesting and impressive. We don't get very much eye candy of an Avatar type. I think this is a good thing as it requires the movie hold up on its own rather than because it looks so damn cool. (Avatar, once I finally saw it, was quite lame, though I could see why some Chinese dissidents liked it at least. While the third Transformers movie was probably the best of the three, that doesn't mean it's a great film, and of course, there's the Star Wars prequels to consider here. All of which were quite bad).