I don't think there's a good answer to this, depends on the person really.

I kinda just fell into converting my bike. It went from an old 10 speed to a ghetto single speed to a crappie fixed to a pretty decent fixed. Pluses are that I learned a lot, have the tools and know how to fix it, and fix it fast, doesn't look that great so it probably won't get stolen when parked with most other bikes and I feel proud to know I built it and have set it up just the way I like it.

Negatives... probably cost more in the long run, especially if you include tools, and time, it just isn't as nice as the newer bikes cause it'll take a lot of work to get the paint just right...

If you know you want a fixed gear bike, I'd probably go right for a new or used complete bike.