I'll chime in on that one...
Hard to say how it will hold up with long term usage, but I have pretty much been using it non-stop since Thursday when I picked it up and it feels as good as new. The sliding mechanism is tight and the two halves to the phone are so close it really limits the amount of movement during use or when bouncing around my pocket.
My girlfriend has the black/blue HTC case we picked up at Radio Shack with the phone it really added another level of stiffness to the keyboard. I guess this could be viewed as good or bad pending on your personal opinion. On one hand it really tightened up the slider, but I could see someone saying it tightens it up too much. It definitely requires a noticeable amount more force to open the keyboard with the case on, which is too be expected with most cases.
That being said, I will probably be picking up a case/screen protector in the near future. Better safe than sorry and all that. The only reason she got the first one is because we both agree that she is a bit harder overall on her phones so we wanted to get that one protected ASAP.