I've been shopping for a PDA/phone hybrid for a long time:
I got the 6315 which is a *good* device, and is on the verge of being great, but still has a ways to go. It was too bulky, the kb was tough to use, the phone signal was flaky, etc. etc. Common 1st gen problems I'm sure.
One of my biggest gripes, that I doubt will be fixed anytime soon is the keyboards with these hybrid pdas. The XDA/MDA III promises a small slideout kb that looks cool, but not really all that easy to type on I would imagine. Combine that with the fact that it is still too bulky for a pocket and an unknown release date, I'm making the sk plunge. The kb is super easy to type on, and since it will be mainly for e-mail/text messaging on the run, I think this is one of the most important features to me.
I've finally brokedown and purchased the sk2 after seeing my younger brother's upgrade from his 1st gen color sk. This thing is a pretty sweet device. My only reservations are the fact that it is a proprietary OS, and there is no bluetooth/wifi. The plus side is they have a very quick data network based on GPRS digital mobile signal (near 56k dialup speed). The lack of BT/WIFI is a bit disappointing, but there are lots of workarounds, and if I REALLY need a quick connection via wifi it's not that much work to fire up the laptop right?
The kb feedback and form factor of this device is tops BAR NONE and will not likely be improved any time soon. The 2nd version can ACTUALLY fit in your pocket and not make you look like your easily excited.
There is also a great developer network with a wide variety of applications available and constantly growing with a majority of applications available for free! I think this beats paying for all those little pocketPC add ons. I guess it would depend on your specific needs, but I really think I can't go wrong with this device.
I'll keep ya posted with further reviews, but I'm psyched to get this thing.