Since I have not seen it very often and found it pretty hard to find decent information about it, I'll share some information about my newest gadget: A T-Mobile MDA.
So - what is it? It's a combined PDA/GPRS cellphone running MSFT's Pocket PC 2002 Phone Edition. Basically, it allows you to make phone calls, sends SMSs, manage the usual personal information, browse the web with really decent speed, and install anything that runs on a Pocket PC (and that's quite a lot).
So - is it any good? Yes, I think it is. I have been a long time user of Nokia Communicators (both the 9110 and the 9210), and this thing works a lot better for me. I know that it's really un-nerdy to say so, but I think that Symbian OS sucks. (I hate to say that since I really hate admitting it when something made by Microsoft is actually better than the competition - but in this case I have to say it is.)
Most importantly for me, I have found a software called Cadenza mNotes which allows me to synchronize e-mails and contacts with our Domino infrastructure. The Nokia communicator was able to access e-mails via IMAP and POP3, so I could have done that. The problem was that I'm a long time T-Mobile customer and they simply don't support HSCSD, just GPRS - which is in turn not supported by the Communicator. Apart from that, I find that being able to synchronize addresses is by far the most important thing for me.