Good
It's small, stable, and user-friendly. The only problem I had on startup had a fix (which indeed fix the problem) that was clearly explained on the welcome screen.

Bad
My only complaint is that clicking the little "x" and then choosing "Close" to kill search windows is a pain. Make it so the "x" closes the window (just like hitting "x" on a Windows window would do). Perhaps add a toggleable prompt ("Close this search? Y/N").

Missing

1. MP3 birates - It'd be nice to be able to see the bitrate of an MP3 file before downloading it. Additional functionality (lacking in most other P2P clients) along these lines could include an indicator for VBR (as opposed to CBR) MP3s and their approx. birates.
2. File types - Would be nice to see a more advanced type of Media search selection. Maybe the dropdown box could include a (customizable) list of common file extensions along with (in parentheses) the names currently used. For example: *.mp3 (MP3), *.ra (Real Audio), *.zip (Zip Archive) *.rar (RAR Archive), etc...
3. Advanced mode - One of the stated goals of XolloX is ease of use, and that makes it great, because it is very well done without being too simple. However, it'd be nice to see some more advanced options (as long as it doesn't add too much to the program's size, I like having it able to fit on a floppy) such as server connect configuration, advanced search parameters (see 2), etc. "Simple mode" should be exactly the way XolloX is today, but there should be a toggleable "Advanced mode" with some more technical options for more experienced users.