Almost all of the battery powered ones with speed control work very well. The most recent I have is really good. I can slow it down to just a couple of rpms! Doesn't do anything useful at that speed, but the point is I can certainly crank it down enough to work with plastic.
I have two Dremels on my bench now- an AC model (still with speed control) and the DC model. I use the DC model normally for most quick work, reserve the AC model for big jobs (like wood work on scratch models) that would take the battery down too fast on the battery/DC model.