I have used SFX, UKRE, and the old DAE Europe with success. Sadly, DAE Europe was ruined when it was taken over by the RCI Borg. I have considered using Hawaii Timeshare Exchange. UKRE only handles exchanges of timeshares within the British Isles, so unless you own there and want to go there, it often does not work for most Americans. I really liked the old DAE Europe, as I got a lot of great exchanges with them, the fees were low, and they were very easy to work with.
I bailed out of RCI when they got into all this points mess, both RCI Points and "points lite" (TPUs), as the exchange valuations became very skewed and no longer honest, plus the fees just kept going up. Also, this is when RCI became "Rents Condos Instead" and started renting out too much of their prime deposits. We had an RCI employee posting here on TUG, Bootleg, who confirmed from the inside that this was going on.
The loss of DAE Europe and the changes at RCI have meant that our timeshare ownership is now limited to a couple of weeks that we use ourselves. The exchange game is no longer what it was, but was very nice while it lasted.