Cheap? Yeah. Fun to ride? Yeah. Great for fitness? Yeah.

One other benefit that my be less evident is the mechanical know-how you gain from putting one together. It makes the most sense to convert an older road bike to a fixed gear by putting on a rear wheel with a fixed or fixed/free flip flop hub. And in doing the conversion, adding, tuning, and removing unnceccesary parts, I learned a whole lot about bike mechanics. I don't know how much you know about them now, but this should be factored in.