Very few people do these days. Prior to the 1950's and 1960's, the only way to get a large roasting chicken was to caponize. You're making the chicken equivelant to a steer in cows.
But, since that time, the genetics going into broiler production imprved immensely. We now hvae birds which will be larger than capons could ever grow in as little as 8 weeks. Modern broilers also have better body conformation and FCR.
The caponization itself is actually surgery (the testicles are internal). So, I feel putting the animal through the stress, to get results which are worse than a custom bred broiler, doesn't make any sense to me. So I elect not to caponize and I can't think of anyone in here that still does (although I've read some stories about people doing it in their youth).
I found a capon once at a high end meat shop and ate it. I'm glad that I did, in the sense in keeping the tradition alive. But, honestly, it was no better than your standard organic chicken and if anything I didn't like it as much. So, I wouldn't buy one again.