I'm testing a Hewlett-Packard iPAQ h6315 with service from T-Mobile USA. Overall, I've had a very good experience, but I have run into a glitch or two. One problems I've seen is that the mobile phone component of the iPAQ will occasionally disconnect itself from the mobile phone network when it goes to sleep.
This is one of the problems that an unofficial ROM patch has been published to fix. The problem with applying this patch is that H-P and T-Mobile will not provide support for iPAQs to which this patch is applied. At least one article ominously warns that "you will do damage to your iPAQ if you install this ROM".
Here is another problem I have with the using this patch. I've looked at the website where this patch is posted, http://www.lvlolvlo.net/, and there is no indication of who wrote it. It's easy to lookup who the owner of the site is by doing a "whois" search, but I don't like the effort that the author has expended to preserve anonymity. I feel that, at some level, you have to trust the developer of software you install on a PDA. How can you trust someone who provides so little information about him or herself?
Comments I've read from some other iPAQ h6315 users indicate that they are willing to take a risk to correct some glitches they've experienced with this device. I wouldn't feel that way if I paid $500 for this iPAQ.
I'd recommend waiting for H-P and T-Mobile to issue an official ROM patch. When a manufacturer like H-P goes to the lengths it has to warn users to avoid an unofficial ROM patch, I think the best bet is to heed such warnings.