Well, actually Evangelion is derivative, and I happen to like it. Besides being the usual "boy pilots his father's robot" it borrows somewhat (but not as much as people exaggerate) from Space Runaway Ideon. It's not as nearly derivative as people make it out to be, maliciously, and with hyperbole.
Besides which, there are several anime by Gainax better than Gurren Lagann. Ebichu, Nadia, FLCL, the Daicon shorts, and Otaku no Video come to mind. Not that Gurren Lagann isn't great, it just isn't the best Gainax has to offer. Neither is Evangelion.
I don't understand fans that are partial to one studio over another due to one or two anime, though, or despise another due to one or two anime. No studio is all that consistantly good or bad, in my opinion.