I just bought the PS3 keypad. This is a simple evaluation:

PRO
- It's at thumbs distance
- It interfaces well with the chat
- It has a touchpad emulation. With quite a bad precision but I've found it useful in a couple of cases.
- It is wireless (bluetooth)
- The battery lasts forever

CON
- No backlight. You can't use at night with no lights unless you are a really good "typer" (?)
- The letters are not really clear. Like gray on black
- A bit too small