I am no expert on this subject but I just purchased 2 of the Iriver iFP-190T 256MB for my kids so I have some experience with these types of devices. First the answers to your questions:
1. Radio works great. No different than any other portable radio.
2. Depends on the length of the songs but you can probably fit atleast 50 to 75 songs on the device.
3. The memory is not expandable on the 190 model and I'm fairly certain its the same on the 390.
4. I usually do very extensive homework before I buy anything and the Iriver was at the top of my list for a portable mp3 player.
Another suggestion I would upgrade the firmware on the device so you may use it for data as well as mp3's. Also, once you upgrade the firmware you can use windows explorer to copy and delete mp3's to and from the device. It is much easier than using the iriver software. Check here:
http://www.iriveramerica.com/support/ums.asp