Went from 4.2.1 to 5.0.1..
Battery life seems about the same...I think.
Reason I say that is because the first day and the next after the jailbreak was released, I was heavily playing with every aspect of iOS5 and the JB tweaks (new and old) all over again like it was a brand new phone, and was just killing the battery.
Now that I've got it how I like it, battery life seems about the same. Overnight, on 4.2.1 it was ~5% battery loss, and now on 5.0.1 it's ~8-10% (so far). This from a iP4 bought a few days after release day (1.5 year old battery, IOW).