SM2 and EZGUI look awesome. Just so you know, Playmaker does optimize OnGUI with version 1.1. In Unity there seems to be a setup cost to calling OnGUI, so the cost increases with the number of objects using OnGUI. The PlayMakerGUI component centralizes all Playmaker OnGUI calls under one OnGUI call - so it's a LOT cheaper with lots of objects.
PlayMakerGUI also lets you preview GUI actions at edit time, without running the game. This can be very helpful for simple UI screens or mockups. But for more complex UI, a dedicated UI tool is probably the way to go... Anyway, give the Playmaker actions a try if you have time - I'm interested in feedback.