Gave the i8910 what i consider a good workout today. My usual 10 mile weekend walk (finishing at a pub of course...).
When i'm walking i have Viewranger (a topo map app) running. It uses the internal GPS. I also listen to podcasts, and check the web when i get to the pub. Occasionally i'll bring up Google Earth to make sure i'm where i should be (topo maps can be a little unclear at times). The screen is not on all the time, but about every 20 mins or so for a few seconds as i check where i should be going next.
I'll also use GarminXT to drive to where i'm walking and get back home.
So, today i was using:
Viewranger
GPS
3g
Media Player
GarminXT
All for about 5 hours or so.
On my E90 i'll usually get home with about 75-80% of the battery remaining, or about 4-5 bars left. Even my backup Sat-nav phone, an old QTEK 8100 with Memory Map and TomTom will get home and still say 82% (though i only really use Memory Map on that phone).
The i8910 got home today with one bar left, the E90 had 4 bars. Both were running Viewranger and Garmin at the appropriate times, next to each other. The i8910 was also playing Podcasts (not a huge battery hog, imo). They both have 1500Mah batteries.
Make of it what you will, but it seems to me the fastest moving app on the i8910 is the battery indicator.