I sprung for a couple of these for the kids for Christmas. The models I purchased are marketed by Nike[techlab as the [play120 (yes, the brackets are in the name) but they are manufactured by RIO. The selling point that got me on the Nike is the convenience of several methods for attaching to your person. You can put it on an armband or a belt clip, or just slide it into a pocket. Perfect for snowboarding, skiing and jogging using the armband. The headphone cord has a remote on it for volume control, EQ settings, and track selection, as well as a digital display of the track number and song title.
As Jaykapur stated, music can be stored as MP3 or WMA files using RIO's software, 64 meg without an expansion card, 128 meg with on the aforementioned units. The software is pretty user friendly, and performs file conversion in a variety of bit rates, from 64 to 320 kbps, so that you can customize the quality and quantity of music stored on the player. They quote 2 hours of stored music without the expansion card, but I believe you must use 64 kbps bit rates, and to be quite honest, the sound is horrendous at that bit rate. 128 kbps is the minimum I would recommend, which yields about 1 hour of play time. Battery life on a single AA battery is about 5 hours, but again they quote it as 12 or so. They must set the volume at whisper levels and not use the little remote (which can be removed to obtain longer battery life) to get 12 hours. One player that I purchased completely stopped functioning, emitting chirping sounds instead of music. Some kind of DAC glitch I guess. My dealer replaced it with no questions asked. I wouldn't recommend buying it mail order or through the Internet because of this reason. I know your thread doesn't ask this question, but I also don't recommend Sony units. They use proprietary software that encodes the music into some funky non-MP3, non-WMA files, as well as requiring a specific computer operating system to run their software, I believe it was Win 95/98 for the model that I originally purchased.
Overall, I truly don't care for the things, but my kids use them all the time. Very convenient when traveling to keep them busy (and quiet).