First the hard plastic cases that are coming in 2 parts. There are the glossy ones and the trubberized ones in a wide variety of colors and styles at ebay or Amazon. The quality is OK but not perfect. What I like is the good protection in combination with the acceptable increase of size. The problem is that you nearly can't remove the front part of the case without breaking the very small bars fixing the case to the device. I like the rubberized ones most. They are looking more stable to me than the glossy ones. With the case it's more difficult to slide out the QWERTY keyboard. But it's acceptable. What I like very much is the protection of the on/off button and then loudness buttons. Because without the case I öfter press these keys withooany will.
Then there are the leather cases. There are horizontal and vertical ones. I tried to use the ones for the Samsung Galaxy S2 also, they fit. But of course not together with the hard plastic case together. For that I will try cases made for the HTC Sensation XL. I will share my experiences as soon as I get the cases.
The third category are cases that are blocking the hardware keyboard. That unacceptable for me. I tried rubber cases for the Samsung Galaxy S2 and it worked for one but the fitting is not not really good. The thickness of the device is quite different from most other smartphones.
Hope I could help a little bit and will share my upcoming experiences.