Well, I'd only heard that it had a lot of filler. After watching it myself, I disagree with it being "filler". It definitely has good plotlines and all of them were penned by the light novels' author anyway, so, not filler.
And yes, that finale was incredible. Remind me to never make someone nicknamed "The Railgun" angry.
Incidentally, Index's story starts a couple of in-universe months after Railgun ended, IIRC. Railgun's recently-announced season 2 will take place during the first few episodes of Index season 1.