Thing is, userspace networking is a lot like the GPU business. Often times, the software is free or even OSS, but the hardware is proprietary.

I know Intel has software for their NIC's to enable userpace networking, but I don't think any HFT uses it. It may be all right for hobbyist experimentation. I left the industry about a year ago, so my information is a bit out-of-date, but my former employer used either Solarflare cards with the OpenOnload stack (very good cards and awesome software) or the Mellanox CX series with VMA stack (amazing hardware with mediocre software. VMA is now open-source, so perhaps that situation has improved).

Note that these cards are a few thousand dollars each, so out-of-reach for hobbyists.