There are many many posts on here concerning the use of iPAQs, Bluetooth/wired GPSs, and MemoryMap etc. I use an iPAQ 5550 with a Rikaline Bluetooth GPS and MemoryMap. The iPAQ's battery lasts approx 3.5hrs without charging and the BT receiver lasts approx. 7hrs.
I cannot rate the unit highly enough. Sure, I've been through the scenario where I didn't charge up the BT receiver before flying so it decided to stop working but I still keep my DR skills up to speed so it wasn't a drama in the slightest. I still keep a flight log, I still pre-plan everything, I never take things for granted, but GPS is great and makes navigation so much easier - much less workload and you can tell passengers what that crumby little village is called by switching to an OS map by pressing a single button to zoom in... great stuff!
MemoryMap's website doesn't make it crystal clear exactly which Navigator package you need to buy and the software needs a few tweaks so it isn't without its faults. Their support team aren't receptive to suggestions either which is a shame. You would think they would welcome suggestions from people who use it in 'anger'. Sadly not though.
Good luck with your purchase!
lhd