I used Smart Actions before Juice Defender and I must say that Juice Defender is pretty amazing. With Smart Actions I got a solid day, with Juice Defender I get basically two days.
There's more options and customizability with Juice Defender, which I like. However, as another person has mentioned, the time to reestablish connection can be a bit annoying.
I do recommend it though. Keeps your battery conserved when you need it later.