I have been using the Tmobile MDA for couple of months already. Just like many of the SDA and BlackBerry I have used, there are cons and pros, which I will highlight them below.
Package Contents: A brand new Tmobile MDA includes the MDA itself, a stylus, SIM card, a USB sync cable, a charger, a belt clipper, a steorea headphone with microphone, and some documentations. If you purchase it from Amazon, you need to call the customer service to activate your phone and service. You will be pre-assigned a phone number, but you don't have to use it if you already owned a Tmobile number.
Price: I got them for $99 off Amazon after two rebates. The deal might not be available now, but I do believe they are coming back pretty soon. The price is certainly worth every penny for this "PDA." I received both the Amazon and Tmobile rebates in about 9-10 weeks time.
Tmobile Voice and Data Plan: You are required to sign an one year contract on a $39.99 voice plan in order to qualify for the rebates. Most websites, if not all, would also sign you up for a $29.99 data plan which gives you unlimited access to the Internet through your MDA and unlimited HotSpot Wifi access at various locations such as airports and Starbucks. However, I have heard many people who successfully cancelled their data plan subscription without any penalty and still received their rebates back. Also remember that you need to pay a $35 activation fees.
Design: The design is OK. I always carry it with the belt clipper because it won't comfortably fit into my pocket. Please remember that this phone is designed for two-hand use, I found it extremely difficult to use it while I'm driving.
Keyboard: The Tmobile MDA features a full unique slide-out QWERTY keyboard. Personally I love the keyboard! The space between each character is big enough to allow my thumb to comfortably "type" fast. Now I can actually send a long email while I'm away from my office.
Battery: The battery is excellent. I normally charge the MDA at the end of the day, but it can easily last for 2/3 days. One great feature: Unlike many other PDAs that require you to have a backup battery in case the main battery is out, you will not lose any of your contents on this Tmobile MDA even though the battery is exhausted.
Windows Mobile 5.0: I have never really been a fan of Microsoft Mobile family. The OS is OK but certainly can have a lot of improvements.
Email: You will be able to download your emails from free services such as Hotmail, as well as POP and IMAP servers. However, you might need to register your email address at My-Tmobile. Otherwise you might not be able to send outgoing mails from the phone. It seems like a common problem yet I can't find a fix on this. The basic idea is: you will login to My-Tmobile and "register" your email address, server, username, and password. Then, you will open Outlook to download these settings into your phone. Everytime when a new email arrives in your mailbox, My-Tmobile will download then send it to your phone.
Web Browsing: It's not fast, it's not slow either. You will be able to surf most mobile-enabled website relatively quick, but it will take some times to download contents from a regular website. However, I am having a lot of problem accesing Google website and all of its services (maps, news, etc).
Updated as of July 12, 2006: The problems dealing with Google seems to be solved and it works perfectly fine now. Note that Google Map will not work on this MDA and there will not be a fix in the future for this version of MDA. Java Mobile does not provide individual installation, it is however installed by the manufacturer. The Tmobile MDA does not have this capability as of now. Perhaps the next version (MDA Pro) will have that implemented.
Modem: Yes, the modem thingy works! If you are thinking to use this phone as a Internet modem to your laptop, it works (with a little trick that you can easily find online). I have often used this feature and the speed is resonably fast.
Bluetooth and Wifi: Not only the phone is not designed for one-hand use, but also it is not designed for talking without a headset. I would highly recommend you to get a decent Bluetooth headset. As for Wifi, it's a battery killer. I hardly even use the Wifi since the areas I work and travel have Tmobile EDGE network. So I wouldn't comment much on the Wifi features.
Instant Messaging: Yes, all IM will work include Skype without any noticeable lag. Skype voice call will not work, at least I have always experienced a few seconds lag.
Camera: I wouldn't recommend to buy this phone for any camera usage. The 1.3 MP camera with 15fps video produces very low quality image.
I certainly hope this review gives you a basic idea of the Tmobile MDA. Personally, I love the phone and it has helped to make my life a little easier especially when I'm on company travel. This review is by no mean a comprehensive one, I only pointed out some of the features that I frequently use and have some knownledge on.