Serpent is (in my opinion) so much easier to learn — and has so many fewer features (this is a good thing, as far as security goes) — that I really don’t understand why it was abandoned by almost all Ethereum devs in favor of Solidity, which is more complex (and therefore, harder to get right, especially for newbie devs) in every respect.
Heads-up, Ethereum devs: Serpent is terrific! It’s extremely simple and stable, and was designed by none other than Vitalik himself! Two of the largest Ethereum dapps (Augur and EtherEx) use it and have tested the hell out of it, and have declared it to be Super Extra Awesome!
Maybe Solidity should be the go-to language for advanced developers that really need (and know how to safely use and audit) its extra features and fancier type system. But I think most devs on most Ethereum projects would truly be better served by Serpent, because it is just plain simpler.