I've found that my bar for investing is the same no matter what kind of company it is -- how quickly can they sell me on investing? As much as us engineers hate to admit it, the most important skill for a founder is the ability to sell. Besides the obvious of having to sell to customers, they also have to sell their vision to VCs, sell their company to potential employees, sell their vision to journalists and potential customers, etc.

If I'm not convinced to invest after one meeting, I usually don't invest. On the other hand, if I'm convinced after five minutes, I'm usually asking where to send the wire by the end of the meeting.

Armory is a good example of an open source/open core company I did invest in. They had me convinced in just a few minutes. They had an answer for every one of my objections and made me feel like I'd be missing out if I didn't invest. And so far they're doing really well and it's one of my best investments to date.