Yeah mp4 (mp4 videos at least), mpg, avi, flv, divx, wmv, f4v and Matroska all work great, havent found any that have failed, tbh I havent used it for music yet but I doubt it would struggle, even if it did its still worth having just for the excellent video controls, playlist is perhaps a bit basic although I havent really explored that and it's options.
It opens fast, always feels stable, drag resizeable, zooms in easily to any level with hotkey, can slow or speed playback speed by any 5% increment between 5% and 300% on all the above files with no problem.
If you set to allow multiple open at same time in options, and select never on top, you can have loads open at once playing different files and click on any to bring them to the front, see the video on the link if you want full video in video views. The interface is minimalist so it doesnt take up too much space yet very functional, particularly if you find the skin option and change to built in skin (which I prefer) over the default. Can set A and B points to repeat between. Um it really has everything I can think of needing.
It's so good it's hard to believe it's free! If it wasn't I'd still want to buy it over anything else.