I've had the Duo of just over a week now and thought I'd pass along my thoughts on the device so far.
1. Form factor: Very good. Not as bulky as a 8125 or 8525 but still plenty of room on the slider-style QWERTY keyboard. Because it's a dual-slider phone, it also has a full size number pad for making calls or sending T9 text messages. It's 7/8 of an inch deep, so it's not as thin as my old BlackJack or the Motorola Q, but it's not as tall or wide as either device so it's, for me, a good happy medium. It definitely fits in my pocket without becoming too bothersome. I also like the fact that the lock/management button is right on the front of the phone. Press and hold it to lock the phone and tap it quickly to change your profile type and manage your connections like data plan and Bluetooth. (More on that later...)
2. Weight: It weighs in at 4.5 ozs, which is just lighter than a BlackJack with an extended battery at 4.4 ozs. It's got a nice sturdy heft to it, it doesn't feel like a toy phone, but isn't so heavy that you can't carry it around all day feeling like you've got a brick in your pocket.
3. Sliders: I was most intrigued by the dual-slide functionality of the device. It's actually got a slider for the keypad/T9 keyboard and a full-sized QWERTY keyboard. Both handle very well, especially the keypad. The only real complaint I have about the QWERTY keyboard is the keys don't have any bounceback to them and they're flat in relation to the back. What this means is that you can't really tell if you've actually typed a character or not. Something else that bugs me is the fact that the delete and enter keys are right next to each other on the keyboard. Bad design in my opinion... One other really cool feature that's missing is the ability to answer/end calls by sliding the keypad open or closed. You actually have to press the answer and end buttons to do so. Definitely not a deal-breaker, but it would be sweet!
4. Battery Life: About two days with light usage. I don't tend to make many calls from the device as I'm desk bound during the workday and find myself using it primarily for checking emails after hours and the occasional web browsing. That said, if you're a Bluetooth user, be aware that the battery will last you less than 12 hours. I'm not kidding. Bluetooth sucked the battery dry in less than 12 hours. I'd highly recommend getting a car charger if you're mobile and rocking the wireless headset.
5. ActiveSync: This is where I use the device most frequently. Sending and receiving messages from the device is great. It's running Windows Mobile 6.0 which is a definite leap up from WM5 but nowhere as robust as BlackBerry or GoodLink Mobile Messaging (which will run on the device).
6. Text messages: My only beef about WM6 devices is the lack of the ability of deleting text messages in bulk. You have to delete each message individually, which sucks when you're getting text alerts about server issues and have to manually delete dozens of messages individually. I've heard tell of threaded text messages with WM6 but I don't use my device for that so can't give a personal review of that.
7. Camera: It's a 1.3 megapixel camera, so not the best thing I've ever seen as far as quality, but it's good for taking the spontaneous picture of your daughter doing something cute.
Overall, I really like the Duo and am not missing my BlackJack at all.