DAP Review has done a nice comparison of the voice recording capabilities of 4 popular devices (the iRiver H-series, the XClef HD800, the Neuros 2 and the Dell DJ). The versatile iRiver was the winner:
Sound Quality of the recording is excellent, best of the bunch by a good margin. Recordings are fairly life-like, clear, and loud. The built-in mic is sensitive and can easily pick up voice from across a room.
The one thing that hurts the results is the hard drive spin-up noise, which is picked up by the mic. It spins up every so often to save audio to disk. The noise is more audible on the iRiver than any other player.