Supported formats
It’s, theoretically, compatible with several sound and video formats (see their list at the above Wikipedia page). In practice, playback is still hampered by the, in cases, VERY bad performance and compatibility.
In addition to my H.264 (video) playback tests, I’ve made pretty thorough CPU usage tests with the most important audio formats it supports. The following figures have all been measured on a Dell Axim x51v running at 624 MHz. As can clearly be seen (particularly if you take the MP3 playback CPU usage into account), the results aren’t particularly good – alternative (better) players all have lower CPU usage. See THIS for more information. Note that 11% at 624 MHz translates to 33% at 208 MHz – the forced CPU frequency in the tests used in the linked document.
Flac: 5%
M4A (AAC): 16%
Ogg: 30%
MP3: 11%
WMA: - (incompatible)
(I’ve used my standardized audio test files – they’re the same I’ll use in my forthcoming Multimedia Bible.)
Note that while the MP3 playback “only” requires about 11% CPU time at 624 MHz, it still has severe problems when running in the background: it stutters when there’s some additional CPU usage (for example, you open a menu or anything). That is, it’s basically useless when not run in the foreground, with nothing else running.
Video playback is much-much worse and practically useless, unless you try to watch really low-quality, low-resolution and low-bitspeed videos. TCPMP or, even better, CorePlayer (the latter is an undisputed king of video playback on all mobile platforms) are way better.
Finally, RealAudio steaming (one of the best features of even the current, initial MPlayer release) is a CPU hog too and is pretty useless on slower models (read: you must overclock your ~200 MHz TI OMAP CPU’s if you want to listen to RealAudio radio stations).