yep, they are usable! with cpu < ~133 MHz you will have to change to 22k sample rate for realtime processing. I grabed video input and used it for triggering samples (about 10) on a 166MHz. Marius.
I've run a Pd patch which controlled 14 samples on a Pentium 133 running Win2k and a Mac 8600 with a 250 MHz 604e running Linux. While you can't do a lot on those old machines, they are definitely usable for some
things.