I've enjoyed many things about this product, including the fast wireless connections, multiple input methods and the bright, clear screen. However, the bugginess of Windows Mobile XP is extremely frustrating and power management issues are annoying.
With Mobile XP applications don't really close when you close them, they are just minimized and still using resources. In order to truly close them you must go through a few clicks to the equivalent of Task Manager and close the applications. Also, with some applications I've had problems with an application getting lost in the background and it won't come to the foreground. The only solution is to reset the iPAQ, which usually means losing what hasn't been saved.
From a power management perspective, I've had two problems. First, an after market battery I purchased doesn't interface properly with the battery monitoring in the iPAQ and it never shows full charge. It starts reporting that the charge is low even though it will continue working for a long time. The more annoying problem is that the iPAQ keeps turning itself on. I'll turn it off, leaving sitting on my desk, and it turns itself on, consuming battery life. I haven't done much research into solving this problem, but will have to soon. I didn't buy this thing so I would have to spend hours researching problems.
The power button is also very vulnerable to being accidentally pushed. I bought a nice leather case for carrying my iPAQ around and found that when I put it in a duffle or on my hip it was getting pushed. I can't always testify as to when the button was pushed or when the iPAQ was turning itself on, but I wish HP would have designed the power button to be a little better protected.
Other than the bugs listed above, this is a very cool product. The wireless setup is simple, lots of good software comes with it and there is a ton of software available out there. Interfacing with your desktop/laptop PC via USB or Bluetooth is also very simple. Another warning ... although a mobile version of Excel is provided, do not expect it to work with complex spreadsheets developed using the big boy version of Excel. Lots of formatting and some formulas won't work on the mobile version. It is great, however, for your everyday spreadsheet apps.