I have successfully got pNunit (http://www.plasticscm.com/infocenter/technical-articles/pnunit.aspx) working to run my specflow tests in parallel.
I really don't like the fact I have to specify that the specific tests I would like to run on each agent though. Tests change and are added frequently. I wouldn't want to have a test not run because someone forgot to update the config file.
In an ideal world, I would like to say Agent 1 would run Test fo Category A and Agent 2 would run Category 2.
Is this possible?
We had a similar need, on CI server, and were not happy with SpecRun, so we built our own tool for it. https://github.com/uShip/Concord
We have it running 130 categories between 12 threads, both locally and server-side. We also have it running selenium tests in parallel.
I personally never tried nCrunch or pNunit for specflow tests, but the guy who started this tool researched it for weeks before just starting this tool. Then I helped enhance the tool for further needs.