I've been on Koh Phi Phi Don and surrounding Islands tours 5 times over the last 8 years and Simba is the best tour operator by FAR. The quality of staff and their detailed explanations of the sights is excellent. The day is carefully and not too busily planned...drinks and fresh fruit are in plentiful supply as you move between islands.
It is difficult to describe the beauty of the Islands and Phi Phi in particular is to me the number one natural wonder of the world - I never tire of going there. Yes, the Islands are crowded; yes at times litter is disappointing (instead of complaining be a decent human being, pick it up after the lazy neanderthal tourists and bin it!!!)...to let these considerations stop you from a world class tourism highlight would be a travesty! Simba is the best way to experience this.