Have tried it with an Olympus - it works and it's useful, but the IR on the N900 is nowhere near as strong as an actual remote unfortunately. An actual remote will bounce off walls and be picked up by the camera, whereas the N900 has to be pointed straight at the camera's sensor to work.
So handy at a pinch, but not something you want to use for anything more than messing around.
Not to mention that the hit area of the widget is microscopic - why not make it a fullscreen app?