You know, I almost -never- see a movie on its opening week.  I hate dealing with the crowds, not to mention paying full price.  However, this past weekend, it just worked out that after repairing my car, I could easily make a showing of "Avengers" just across the street at a matinee price.  

...despite the crowds, despite the flocks of unruly children and their inconsiderate parents...  I'm glad I did.  
I seriously haven't had this much fun in a movie since...  I can't remember when.

Mind you, I'm no "Avengers" fan girl.  In fact, I'm really only familiar with them through the recent revamps.  I believe I was dimly aware of their existence previously, but remember always being terribly confused as to why Thor was involved and why he was being so... polite...   I always thought of Thor as a cross between a frat boy and ...mm...  Bear Grylls.  Overall, I grew up more with Batman, Superman, and the X-men.  So, really, these characters and their universe are all new to me.

...and yet, even though the only Avengers movies I had seen were the Iron Man series and Thor, I had absolutely no trouble understanding what was going on.  I would suggest knowing the basic stories to Thor and Captain America before going in, but I really don't think you have to.  You'll enjoy it anyways.

When I say it is a "fun" movie, I really mean it.  The humor and action are fantastic.  It has moments of drama too, of course, but nothing like the grim existential crisis of the revamped Batman series.  It's a musical, accessible and bright, versus Dark Knight's operatic pathos.   They're both great, both deliver a satisfying experience, but are at totally different ends of the spectrum.

So, yeah.  Whether you're an Avengers fan or not, you really should go see this movie.  Even if you need to do a little wikipedia reading, you will have fun!