No background process like mysql or pgsql. Thus helps me run on cheap ram. _IF_ it grows and gains traction, then switch to a better db, which until now has not happened. So I haven't thought about the "better db" i mentioned above.
However, RF does about 80,000 pageviews per month, has over 1000 registered users and gets about 10 posts a day, and SQLite hasn't proven a problem whatsoever. It's hardly a big project but it's bigger than 90% of small, part-time projects that use technologies that are more advanced than they really need.