A minidisk will give reasonable quality recording. You should note the caveats that others have mentioned, i.e. that the quality specifications are not as high as those of other recorders.
How are you going to get stuff into the minidisk? Some have built in mikes (and I saw a neat one that came with a little stereo mike assembly), but these are generally of average quality.
If average quality is all you are interested in, then fine. You may be able to do it even more cheaply by getting a budget cassette player with a built in mike. This could do the job if you are just after an indication of performance; you can easily tell if someone's out of tune, or the phrasing is loose, or whatever. But that's about all.
If you want something that you could use more broadly (e.g. to burn to a CD for a demo), you can get away with a minidisk recording, but you would need to pay attention to the front end. Someone else mentioned a little mixer, which is very sound advice.
