I installed the Treo 600 "Internal Extended Life Hi Capacity Battery 19% More Battery Life" from PDAParts.com in early July. The install went well following the instructions on their site. It's actually easier than it looks and took less than five minutes. I did buy and use their Torx wrench set and the case opener tool.

After over a month of normal (for me) usage I'd have to say I'm seeing in the neighborhood of ten percentage points better battery life in most situations.

The Treo sits in it's cradle most of the day. I may handle 5 calls in the evening, 10 email messages, an hour of e-reading or games, and a bit of calendar/todo/text lookup. Before the battery upgrade the battery percent was in the mid eighties by the time I put the Treo back in the charger the next morning. Now it's rarely below 95%.

Weekends I probably do 15 calls, 20 email messages, an hour of web activity, and three hours of reading. It's never gone below 80% from Friday afternoon to Monday morning. I leave the phone on the entire weekend.

I can't detect any difference in charging time.

Some of this improvement is due to replacing a nine month old battery. Some is due to the slightly higher capacity of the new battery (1800 to 2000 mah). In all I'd say don't do it if you are doing okay with the battery right now. The gain isn't all that much. I do highly recommend this, though, if you have an older unit.

A side effect to replacing the battery was that I re-tightened the screws that hold the case together. I'm pretty sure this made the buzzing go away since the screw behind the antenna contributes to it's electrical connection to the rest of the RF circuitry.