At the first sentence I have to remind people who are not familiar with computer systems should not consider a MID/UMPC. MID/UMPC can be simply understood as a handheld computer for advanced users. The OS re-installation and application installation are not easy. I have picked some considerations for potential buyers.

1. Speed - CPU Type, RAM

I believe speed is definately the first consideration for buying any computers. It indicates how fast the device can respond to your commands. Of course the faster the better, but somehow the speed is positively curved with the price. You'd need to figure out the balance point for yourself. To better understand the actual speed, it is wise to find an unboxing videos online. Special thanks to the founders of Youtube :).

2. Convenience - Dimension, Weight and Screen Size

This part is merely personal preference. Some people prefer bigger screen size to comfortably read their ebooks and movies. Some people would like to buy a smaller device to put it in the pocket. People are likely to be unaware of the weight, I'd remind you that devices with similar size can have very different weight. Metal cases usually weighs 150 grams more than plastic cases. 

3. Usability - OS

Expensive MID/UMPC are usually able to be installed with Windows XP/Vista/7, low cost products would tend to support Windows CE/Linux/Android. Applications are depending on your OS. You cannot install Windows XP applications to Windows CE, and Google Android system can only support its own applications. In addition to 'what OS they support', you'd also need to know what OS are actually installed for your purchased item. Manufacturers can offer you only Linux OS to make attractive price. They can promote it as 'Windows 7 Compatible', yes it is absolutely correct, Windows 7 compatible, but just not yet installed. You can always buy a licensed CD-Rom to install it youself.

4. Connectivity - Wifi, 3G, Bluetooth

Make sure how your desired MID/UMPC connects internet. Usually these devices connect via both Wifi and 3G. Wifi is easy to understand, as long as there is a wifi hotspot and your device can receive wifi signal, it should work. Then 3G is something more complicated. The internet is connected through your mobile phone service provider, and different service providers can have different telecommunication standards (known as GSM, CDMA, etc.) MID/UMPC usually supports both EDGE (3G of GSM standards) and HSDPA (3G of CDMA standards). If you do not understand what I am saying, you can simply follow this:
a. Check if your mobile phone service plan has EDGE or HSDPA 3G service.
b. Check if your desired MID/UMPC supports EDGE or HSDPA 3G standards.
c. Check if your desired MID/UMPC needs an external USB 3G adaptor. Also check if you are able to install the driver of that USB 3G adaptor.
d. Put your mobile phone sim card which is enabled EDGE or HSDPA 3G service to your MID/UMPC.

Thank you for reading my Purchase Guide for MID/UMPC. Please comment to supply more information if you have some interesting points. :)