I used one on a SonyEriccson p800 and it worked well for me. It protected it from falls, was easy to slip on and off and did not detract from the styling of the phone at all.
On the other hand, I heard some say that it feels squishy and may seem to cause an echo (from covering the mic and/or earpiece) on both ends of the line. Note that there are holes cut out for the mic and earpiece but the silicone can slide around a bit and cover one or both.
Here's a thread at Howard Forums where 3vw modified a Martin Fields Silicone sleeve for a belt clip:
http://www.howardforums.com/showthre...licone+%2Bp800