I've used and watched her a bit and she really isn't bad. Her moves are different enough from her sprite base that you won't feel like you are just playing another Satsuki edit, but rather a unique tech character. Set up your attack enhancers, play a little keep away, and than punish them when they are surrounded by enough notes. Her moves make plenty of references to the show, and her Hyper actually uses a clip from the anime which pretty much describes Yui perfectly. Admittedly, she isn't the best character ever and may not be for everyone, but if you are a fan of K-On or even just Yui than this should get you your adorable guitarist fix for the day.
Mind you I'm not the best at figuring out game strategy and I don't watch timing closely enough for things like her notes. My opinion is mostly from observation and a little play time after installing her. So...yeah, grain of salt.