I am currently DMing this with 3 players, Pathfinder initial builds but mostly following 3.5 rules. Group currently is a fighter, cleric, and sorceror. We are just starting Hook Mountain. We have had only one death but multiple players down in many fights. Has been a bit of a challenge for them, but fun. Make sure you let them frequently find potions and I have been using the reserve hp system I found somewhere on these boards to keep downtime reasonable. The only fight I had to seriously tame down was Xanesha. Hook Mountain has the addition of some npc's so this should take some of the pressure off. Overall has run very smoothly.