One thing we did with one project (that needed many VCP "attached" devices to one PC host) was to use standard USB HID on the 8051 end and then exposed or converted these to VCP using Eltima.com's virtual com port software. On the USB side we only needed to write application level software to talk to the USB HID interface and then we simply redirect that data to a VCP for our customer that had a legacy appl that could only use a COM port. It's a very solid solution.

In other projects we have used other VCP drivers, for example the one that Cypress offers for Encore. It works OK, but is very low speed. It compares to maybe 4800 baud.

I think the hardware FTDI solution is also a good approach and we may use it someday. They have lots of drivers and are very aggressive with keeping up to date (x64, Vista, etc). I'm not so keen on Prolific, but some others use it with success.

I was curious about the usbser.sys option too and have posted a query about it on SILabs. I have to say that I don't have much confidence in it, ... though I have no direct experience with it either.

GB