my 2 cents

All of those machines are pretty good.It kinda depends on what you want to do with them. Are you looking for a sketch pad for ideas or to make a finished product? I never have used the boss units but they seem like they have a lot of features that you might not need. The fostex is a bare bones, good for ideas and live stuff but will get the job done. The tascam looks well built but all of these machines have their limits. so it goes back to what you will be using them for....

they are great for demos but they do have their limitations. If you are thinking of something more serious, get a computer daw.
I used to have a roland vs 880 and it was a good demo and band rehearsal machine. Nothing more than that.