You guys decide. It's a good PrC, and it gets quite harsh after ten levels, but then again it is a ten level prestige class, on par with most if not all ten-level prestige classes, or with a straightforward non PrC'd character
(it means letting go of +5d6 sneak attack damage, for instance, which at level 15 with a rogue means 4 sneak attacks at +9/+9/+4/-1 at 1D6+8D6 against a lvl5ftr/10derw +18/+18/+13/+8 at 1D6 and once a day +18/+18/+18/+18/+13/+13/+8/+8 at 1D6
for one round .That means the rogue would do more damage if he hits
once in any round then if the dervish hit
every attack in his once-a-day super slasher attack.
But we all know that by the time either of them got to the enemy it would allready be 'destructed' by the 15th lvl cleric or polymorph any object'ed into a frog by the 15th lvl wizard, to say nothing of the 15D6 maximised fireball by the sorceror
)