Combining Dervish with say, Scout, Unfettered, Monk, and/or Tempest can get pretty sick, especially with feats like Combat Expertise, Spring Attack, and Whirlwind Attack, plus using gestalt rules (which is the case in my game). It's a powerful build, sure, but it has its weaknesses too. The fatigue penalty for Dervish Dance should be taken into account, so DMs make sure you enforce it and don't hand out magic items that prevent fatigue and exhaustion (like Armor of the Neverending Hunt from Complete Warrior) without thinking it through. Also, DDs aren't nearly as effective against a single powerful CR enemy compared to multiple low CR foes, especially one that is larger than the DD. Use such a foe's longer natural reach and abilities like improved grab and snatch to lay the smack and stop the DD dead in his or her tracks. Or keep enemies at a distance from the DD's melee attacks using ranged weapons or environmental or magical factors (foes shooting arrows from a cliff high up, or using flying enemies, caltrops, tanglefoot bags, spellcasters using web, slow, grease, etc.). Also remember that in most cases a DD treats his or her weapon of choice, the scimitar, as light. Have foes try to disarm them (or sunder if you're really that mean). No weapons, no Dervish Dance. In closing, a DD build is powerful, yes, but it isn't the end all and be all of PrCs.