Evangelion has pretty cool mechs and whatnot, but the big thing about it is how serious it is typically, over other series. Evangelion, if you get into the series enough, will make you feel like you've been hit by a freight train by the end. Additionally, the somewhat sharp yet gritty artwork acts as a draw in to the series, and is part of the experience as a whole.
Evangelion is so highly regarded, I'd suppose, since it could be said to be the series that brought anime to a new level beyond just samurai, mecha,and shoujo/shonen stuff.
Definitely watch it.