The biggest issue with groove clips is that the length of the clip is almost always calculated incorrectly.
If your original clip is 16 beats ( 4 measures @ 4/4) be sure the groove clip is also 16 beats. More than likely it is not.
MIDI groove clips are generally ok. It's that audio clips that get flaky.
Speeding up the clips is generally fine. Slowing them down is another story. You can go about 15% before artifacts set in. Beyond that, the effect is fairly horrendous, and you  are better off retracking.
That said, I do like groove clips. You just need to be aware of what is happening.

The reason why I provide both REX and Groove Clip files in loop libraries is that in general, REX files slow down more elegantly, while Groove Clips speed up more elegantly. However REX files work best with percussive sounds - anything sustained, particularly if it overlaps with other sounds, will not work well. Groove Clips will usually do a much better job.
 
I also record files that I intend to use as Groove Clips at 100 BPM or slower.
 
Finally, note that you can create pseudo-REX files in SONAR.

Thanks for all the feedback.