I've got the Buffalo, it's less than perfect but OK. It's a pain to set up, particularly if you want to do something obvious, like have all your emails on the shared network drive. Setting up folders has to be done via the setup systems as well, which is a bit of a pain.
A plus point for the Buffalo is the automatic backup to a connected external USB drive. This is great, as you can schedule backups in the middle of the night and be reasonably sure that your data is kept safe and readily accessible if the worst should happen.
A minus point is the built-in print server on the Buffalo. It's rubbish and falls over with monotonous regularity. If you try and print more than a single document it causes the printer to crash, for some reason. I've found that the only way to print big documents, or multiple copies, is to plug the printer back into the main PC and print directly.
Other than that, the performance is OK. I've streamed video from it with no problems and tend to use it as primary storage for virtually all data files. Once accessed for the first time in any session it seems to be almost as fast as the built in drives.
VP