I'm loving the Rebuild flicks so far. Definitely need to buy the first; I'll probably snap it up once 2.0 comes out in the States. The movies have me eager to rewatch the series, too -- I actually bought the series for the first time a month back specifically for that purpose, although I'm not sure when I'll get to that. It's been years since I watched the show, though, so I'm sure a lot of it will be fresh to me too. Hell, I'll probably rewatch End of Evangelion as well. I saw it only once and barely remember anything aside from all the freaky imagery and Shinji pulling a Bill Clinton on Asuka.
From what I recall, 1.0 is mainly cosmetic changes, aside from the very end. It looks great though. 2.0 is almost all different. There are basic scenarios that remain, but the characters are switched around, and of course the development of the primary characters is different too. Mari also throws a wrench into things, although she is still fairly mysterious by the end of the movie.
As far as I can remember, Kaworu doesn't appear until near the end of the TV series ... there aren't even hints of his appearance before he arrives like there are in the Rebuild movies.