G1 here, hax0red & overclocked.
There are tons of different roms you can run, and it can get pretty complicated.
Check out some of this kid's videos to see if it's something you'd be comfortable doing.
http://www.youtube.com/ItsAndroidsWorld
I definitely recommend doing it though - it's like a whole new phone, and it's keeping my Nexus1 envy down for the most part.
Interesting... i just rooted my phone and I'm experimenting with some different ROMs... I definitely think a battery upgrade would be a smart move for me. I'm using the stock battery and I upped my clock to 528 and the battery level dropped from 50% to 30% after about 2 minutes of messing around with some menus and things...
I got my 2300mAh battery on ebay for about $12, including the "hunchback" battery cover. It seems to make the phone a little more creaky and cheap feeling, but I've dropped it a few times with no damage. It also makes the phone a little thicker, but I actually find it more comfortable in my hands.
Looks like they make a 2850mAh battery as well - I might grab one myself.
All kidding aside...anyone else got one? I got one on the way, should be here in a day or two.
I think I'm going to buy a Samsung Galaxy S, any thoughts?