For years (I think I started in 1998), I carried an HP Jornada handheld PC (the first one literally fell apart so I got a more recent model which likewise was loved to death). I wrote over 100 articles, blogs and white papers on this handy device and took notes at numerous meetings. It was small enough to use on a plane even with the seat ahead oif me pushed back, and unobstructive enough not to put a barrier between me and a customer during a meeting.
It ran Windows CE and Pocket Office with Word, Excel and Powerpoint (view, hide and reorder only). I had PCMCIA cards for Ethernet, WiFi, PGA projectors (for powerpoint); I had an extra battery; I had a VPN client for corporate network access (including email) and could sync up my contacts and calendars with my desktop. Often I traveled without my laptop avoiding significant muscle strain;)
When looking for a replacement, I would have bought another Jornada but this form factor was not generally available (Netbooks have now filled this niche). My needs were simple: small form factor including keyboard; always on; integration with my office Windows environment (i.e. Exchange) and with OCS 2007; and good word document handling.
So I bought an HP iPAQ with a foldable keyboard, running Windows Mobile 6- the latest and greatest. Exchange access and Office Communicator ran like a jewel (and was VPN-less), but, possibly of no surprise to you, Windows Mobile was awkward to use and felt archaic. Even though I was well acquainted with Windows CE, an early predecessor, I still had to learn a new user interface which was neither Windows nor CE. To this day, I still can't figure out how to open two documents at once!
WiFi access on the device was also very finicky, particularly for corporate network access- I couldn't find a VPN client that worked, and 802.11i was very flakey; even guest access requiring a browser-based login didn't work even when I called in the IT gurus.
Caveat: My experience was on a PDA (not a Windows mobile phone) and it's never clear how much was due to Microsoft vs HP.