i personally never used one but you should take into consideration that USB devices are processed by the CPU, CPUs are not designed to process/render video. from my experience with linux, a dual core pentium M CPU is capable of playing 720P, 1080i and probably 1080P if my LCD would support it.
when video is processed by the CPU you should expect the CPU to reach very high temperatures as it is being used at 90%-100% capacity. this is how i burnt the fan on my acer lappy.
you also have to consider the fact that while the CPU is processing video the OS would not be very responsive.
the new ION boards might handle this better because this chipset is designed for that purpose.
from my experience a dual core is capable of pushing 1 output, i believe a quad core will push 2, but i doubt it will handle more then that.