I'd be very curious about the viability of clients who approach you in a coffee shop. Seems like you'd get an awful lot of, "So hey, I have this idea that's like Facebook and Youtube combined and I think it could be the next big thing". That's not to say you won't ever find a good client in a coffee shop, but I suspect it has a lower viability rate than a Craigslist ad, and at least on Craigslist I don't have to spen…
Long story short, I am still working full time at the investment bank I've worked at for the last 7 years but I have also been working 15-20 hours per week for him from home on nights and weekends on side programming projects. This has gone on for 2 years now and is exceedingly lucrative.
Don't discount the happenstance meetings in public like this. Some of the people you meet will be crackpots with an idea and nothing else, but experience will help you filter them out.