I like the new Dropdown and Split buttons. Yay! I was using a Select component to mimic this and it was far from ideal. I am replacing these with Dropdown buttons in a key app as we speak.
I see a significant issue with the split button however. You need to click the arrow to select button, but then you need to click the button to fire. That is two steps. The user will expect that when he drops down the button options and clicks something that the action will happen right there and then, like it does on the Dropdown button. I know my users are going to be frustrated by this so I cannot yet put the spit button in play for a couple of apps that could really use it.
If this is not the behavior others want, then an option to decide how it should function would be even better.
Secondly, I would like to be able to set the Split button's currently selected item programatically. The docs are still not up so maybe there is a way, but playing with intellisense has yielded no answers yet.
The last item is for both. I would like to be able to add divider lines between and color to the items. Here is an example from your Apps page that is a good model.