I dislike the fact that it only plays WMV9 video. No free open-source program outputs WMV9. It is a proprietary format and is one of Microsoft's tools for fostering incompatibility to lock out fair competition. So the video feature is pretty much useless for me. If I cared about video, this would suck, but I don't care about video.
They could probably add a couple of things that would make the machine more useful. First, why not include support for reading text files? It would just be a few lines of code. Second, basic PDF support would be nice. It would be cool to have city transportation maps and train schedules on the media player. Anyway, I like the machine a lot. It does what it is supposed to do. This is high praise, and not to be taken for granted.
Update (July 2008): My Archos 105 has shitty battery life when playing MP3 files. It is much less than the 17 hours that they claim. I think that I am getting about 6 hours of battery life. Furthermore, the battery drains by itself when the Archos 105 is not being used. After a few days without use, the battery is mostly dead. I am disappointed by thing, having gotten used to getting more that 80 hours on an AA battery on my old Panasonic.