I've tested GrowFX demo, now playing with SpeedTree. I liked everything in GrowFX, except UI - when tree grows in complexity, it's really hard to keep track what effector controls specific areas. Other than that it's very solid plugin.
SpeedTree seems to be even more powerful and flexible and its UI is much better, although still not node based. Also it can produce geometry suitable for subdivision, which is great if you want to make hero trees. But it has very poor control over leaves compare to GrowFX. And there's stability issues from time to time.
Next, i'd like to try Plant Factory. From what i saw, it has proper node based UI. Should be much fun.
Edit: btw there's sale on GrowFX, 50% off of regular price.