Saim Hann is pretty good, strong quick, agile. The downside is if your opponent locks down your psychic powers, you're screwed. You can minimize the chances of that happening by taking a second farseer in your jetseer council however.
Serpent spam is... well, how to I put this nicely.
http://www.dakkadakka.com/dakkaforum/posts/list/370531.page
That was Smitty's (And eldar player here on Dakka) experience at a local tournament with serpent spam. As you'll see from that batrep for his three games, that his tanks were next to impossible to crack. That said, it's the most competitive build for Eldar right now.