I have a htc tytn II and the internal GPS is crap, so I use a Holux 1000 connected via Bluetooth and it works like a dream. Battery life is ok and was caching for 6 hours during a 11 mile walk and the battery hadn't ran out. This was while Memory Map was running the whole time. I carry a spare battery just in case, but hasn't been needed yet.
to extend the battery life I have the back light on about 50%
Cheers
Mark