I haven't tried this one yet, but I've tried another immediate mode ui library before and it seemed to busy loop a single cpu core. Is that normal for IM GUIs?
They typically draw/process every frame. So if you're doing a game or real-time tool, it will be part of your main loop. I wouldn't call it a "busy" loop, since usually you'd sleep or yield when done processing the frame and wait for the next frame.
There's no reason you couldn't take the same approach but only process/redraw in response to an event (e.g. on a mouse move or click). But since immediate mode GUIs tend to be used in contexts where you're already drawing every frame (games and interactive tools), people tend to use them this way as it fits the best.
The main defining characteristic of an immediate mode gui is it tries not to store state for each widget. Rather than having a large "object graph" (retained mode), you take a more functional approach and draw what you need every frame. There's much less bugs/complexity trying to keep your Model and View in sync, but somethings that need extra state (e.g. tree view expanded state) get more complicated.
