I mentioned on July 20th how I'd installed a 3rd party extended battery from Seidio in my HTC Incredible smartphone, and immediately began seeing huge improvements in battery life. (Well, I should, given it's nearly 3x the capacity and did add quite a bit of heft and bulk to the phone — but I don't care, as I think it actually fits my hand better now, and I like not having to worry about saving power.)
The last few weeks, I've been dutifully charging and discharging as the battery manual suggested, letting the phone run down until it demanded the charger (<15%). As of this post, it's been over five days since my last charge, with all functions left turned on (GPS, WiFi, 3G, data synchronization, automatic display brightness), and I still have 30%.
On update: With light use only, the phone lasted another day and a half on that same charge. I put it on the charger last night just before I went to bed (2am) at which point it was at 17%. Probably could've made it to morning if I'd left it — which means I'm getting up to a week on each charge. Very, very happy with this battery.
Update: 9/11/10: Just as a checkpoint, the most I've gotten from the battery was 7 days with relatively light usage, the least was 3 1/2 — but that evening I was watching a lot of YouTube vids. Averaging about 4 1/2 — meaning if I take the phone off the charger on Monday morning, I'll most likely be plugging it in again Thursday evening. This is with everything turned on, including GPS, WiFi, Bluetooth, synchronization.
Only just got Android Froyo 2.2 last week, so I haven't yet really learned if there's been a battery improvement out of that or not.
Another detail I've noticed is the first 10% of charge disappears within an hour or two at most, but then it goes down very slowly after that, at a rate of 15-20% per 24 period.
Update: 9/17: Yes, Froyo appears to have been an improvement for battery life, although with a single sample it's hard to be certain. Anyway, I'm at 6 days since last charge, and still have 30%. Not many phone calls, but a fair amount of web activity, one hike, photos, emailing them, and playing blackjack.