I loved Medusa whenever I got her on random. She seems a little weak early on, but after a lot of farm she is really insane. Overall DPS in a team fight is huge with split arrow, and manaa shield makes her harder to kill than some of the most tanky champions. I never really got a good usage out of her ultimate, but she was strong despite that.
I usually built a soul booster first, into a bloodstone, and then worked on manta style and such. Probably a terrible build, but I've only played her a couple times, and it worked.