i'd have to say that the best portable audio recorder i've used, in terms of sound quality and durability, is a sony minidisc recorder. the one i have (an older one,
Sony MZ-R50) is all metal; the MD cartridges are pretty much bulletproof. it records in either mono or stereo; i use a little stereo condeser with it, using the regular ATRAC scheme, not one of the newer NetMD recording modes, which gives you more time per disc. i get 74/80 min per disc, and the quality is excellent. i use it mostly for field recordings used for sampling, or for recording rough music demos/ideas; i just have it recording when i mess around.
can't tell you how well the newer MD units perform, though.
tr