Just to let you guys know, i have a 6515f which i love, but it developed a battery issue about 6 months after i bought it, at first i thought the phone was buggy because it would "brown out" if i treid to run media player or tomtom or anything that requied a bit of juice. I bought a new battery after folllowing a previous thread in this forum, and lo and behold, my issues went away................for 6 months then back to normal, crashing and brown outs. I bought a thicker battery (2500ma) than the standard 1300ma one off ebay from hong kong and i have not had any issues since. my assumption from this, is that htc/HP designed a phone/pda that had too high power requirements for the battery supplied
(hp's official stance on this is that the pins on the battery cover are worn and dont make proper contact to seal the power circuit. You can send it in to them to get a new back (and new battery) but take my word on it, it will do the same in 6 to 8 months.)
NOTE: 6515 does not experience these "brown out" issues while connected to external power source (irrespective of the "bad" case pins)
Upshot of this has been that while my phone now works well, it doesnt fit in my car charger or the desktop cradle because of the thicker battery, reducing its usefullness to me. I am ripping out the sim and buying a p3600i to replace it.
Hope the info about the 2500ma battery helps anyone having "brown out" issues
Edit: the p3600i proved unreliable as a gps so im still using the 6515