I will admit that I was little unsure at first about Prisma Illya considering the manga it's based off is not really that great until the later parts but the anime has succeeding in winning me over with this:
[youtube]http://www.youtube.com/watch?v=MxbN1InMuQM[/youtube]
Do note that this video is from the NicoNico stream version of the episode and not the broadcasted one which had a lot more unique shots and didn't reuse the same couple while also fixing some mistakes.
At the end of the day it still has a lot of things I dislike about it, such as Illya herself not really being the same character from the original and so on, but as long as you take it as its own thing and not an official part of the Nasuverse (which it isn't not being written by Nasu or anything) it's fine.