Battery life after at least flashing a ROM and custom kernel should improve greatly, depending on the rOM.
With Fresh1.1, I still used the auto-brightness, just because it worked pretty well for me. If you put it on half you should still get pretty good battery life, at least a full day.
I can get 18 hours out of my battery easily, with the auto-bright on, and since I charge it every night that's all I've ever really checked. It's usually not below 50% by that point, so I would assume I could get a good 30+ hours of moderate use out of a charge on Fresh 1.1 with gumbo kernel.
Now, on the new cyanogen-like 2.0.1 ROM, I've barely managed 12 hours of pretty much no use on a single charge. It's been off the charger for about 11 hours, and I'm down to 12%, and I think that's just a major drawback of this ROM at the moment, something is keeping the phone from sleeping, and I haven't looked to see what it is yet (pretty sure it's mobile network not going dormant). That's with brightness at like half, and the screen hasn't been on much all day, so I'm going to have to take a look at that sometime.
But, basically, if you flash one of the 1.5 ROM's, you should get great battery life (18+ hours on moderate use), no matter what your brightness settings are.
