Yes.....yes I have. And it was amazing!
First of all I knew that the movie was going to have to try pretty hard to suck for me not to like it since I already love learning and reading about that historical era (Sparta, Greece, Battle of Marathon, Peloponessian War, etc.) so I went in with a substantial set of expectations and the movie blew those away.
I'm sure the IMAX helped, but I saw it in the regular theatre and the visual genius displayed by the Directors and Graphic Animators shown through even there. I also loved the idealism of the movie, an old Babylon 5 quote from G'Kar in the episode "Late Delivery from Avalon" coins the experience perfectly for me.
"They're the bad guys and we're the good guys; and they made a very satisfying THUMP when they hit the ground"
Absolutely loved the flick, I'd say must see for anyone. Although do mind taking the little ones, since they're not kidding about the R rating.