The old wheelset on my bike was on the way out. The rear wheel was breaking spokes at a rate of one or two a month, the bearings had been replaced twice and the cups were kinda pitted, and the rims were getting worn down pretty close to the wear mark.
I was thinking of just buying some more cheap machine built wheels, but said fuck it . . . and decided to try my hand at something new. Bought the rims (Kinlin XR31) and 32 hole hubs (Ultegra 6800), calculated the right spoke length and bought some double butted spokes on sale. I also picked up a Park Tools tensiometer, which wasn't really necessary for building the wheels, but I think was a good idea . . . I'll be able to keep a more even tension on my wheels from now on, which hopefully will make them last longer.
Following the instructions from Sheldon Brown's website wasn't too bad. I did try to rush things initially, resulting in screwing up the front wheel . . . so after lacing it up and then starting to tension things I discovered a problem and had to completely take it apart again. I'd say it was about ten hours to build the front wheel, then about four hours to build the rear wheel. Throw in another couple hours to get everything trued, stressed, and dished perfectly.
While I'm not sure that I saved any money over just buying something ready made, after a couple weeks (about 400 km) riding the wheelset I'm quite happy with them.