I've used Tidy Cat for years because it clumps so well and controls odor and all that. But it's clay-based, so not very environmentally sound. I'm actually going to try to switch them over to one of the grain-based ones in the coming month. I bought two big bags of it (forget the brand) and plan to just slowly attempt the switch-over one litterbox at a time (they have 4).
I tried "World's Best" a year or two ago and wasn't impressed. It was very "dusty" and clumped very poorly. I now refer to it as "World's Worst". Other people swear by it though... *shrug*
As far as transitioning goes, I rolled the dice with mine and just immediately transitioned them to what I wanted them to use. Actually, I tried for a week with the pellet stuff that Neytiri used at her breeders, but couldn't stand it (too smelly and difficult to manage). An abrupt change though of course is a risk with any cat. Glad in my case it worked out