I've used AnywhereMap for about a year - both in the US and in the UK. It is very functional and offers more facilities than anything else I've come across. The US database is excellent - it is frequently updated and the download process is simple. The process is simpler than doing say a Garmin one (which is pretty simple itself). The updates are cheap - free for 6 months then about GBP75/year thereafter.
However, the European database is not nearly as good. The latest version available "expired" in June 2001. That is to say, it can still be used but that was the expiration date belong which is was no longer deemed current. Worse still (most airfields and navaids haven't moved in the last year - some might close and frequencies do change but I always have up to date paper backups) is that the UK database of airfields is really lacking. I'm not just talking about unpaved unlicensed runways either - Elstree isn't in there for example. It wouldn't be so bad if you could add your own, but you can't. You can add waypoints, but not airfields. ControlVision have said for months that they'll release an updated European database soon, but have not done so yet. The European database has no approaches and no airfield diagrams either, unlike the US one.
I've tried quite a few and no other PocketPC software around is as good as AnywhereMap.
Battery life on the iPAQ can be fairly good - with the backlight on a low setting you should get 3-4 hours or better still with the light off altogether as you can when using it in bright daylight. An expansion jacket (essential if you want to use cheap compact flash memory cards) can hold an extra battery which will add 70-100% to this time. As Chuck says, it is easy to rig up an external battery pack or you can buy ready made ones. Not using the aircrafts power sockets (even if it had them) means you are completely independent of the aircraft. They even do an external gyro which will gives you an AI in the event of a vacuum or electrical failure.
Keep in mind you'll need to power whatever GPS unit you choose - the figures I've suggested above are not powering it. The neatest solution here is a bluetooth GPS - no cable to tie the two together and at least 8 hours battery life. Some people may worry about the RF nature of the communications but empirical evidence suggests otherwise.
My personal favourite for UK flying is a Garmin for routing (mainly because of the shortfalls in the database as described above) and a moving map display of a CAA chart on the iPAQ.