I played Alariel, my epic level dervish this weekend, she was a blast. Granted, there were some house rules involved (all gateway feats can be replaced), and some third-party books used (Feats book), but the experience leads me to believe that a dervish is not overpowered. Sure, you can be extremely fast and get a large number of attacks, but your damage output is rather low in comparison to a true tank. Still, Alariel was designed never to get hit except on a natural 20 (AC in the 90s), and to rarely miss (Attack bonus in the 50s and 60s), but unless she's fighting evil outsiders (an evil outsider dread weapon), her damage output is only in the 30-40 range per hit (90-120 range when she crits).