We bought a spec house, but it was just getting underway when we signed the contract. I like that option, it worked out well for us. Got to pick all the finishes. Made some minor changes along the way to the plans, changed the appliance choices to fit my preferences, etc. So I got the upsides of a spec build, but I also got to make sure they put in a properly large kitchen island, double ovens, and other improvements that usually get left out of a pure-spec build.

I also got to visit the site every day and take pictures, which is really nice to have. Especially the day before they did drywall, I went nuts with pictures and now I can tell you exactly what the inside of every single wall looks like, what the electrical looks like, the plumbing, conduit, smurf tubes, etc.

Also, if you are nice and polite, you can talk directly to the subcontractors in some cases and get even more customization at a great price. E.g. the electrical guy offered to run cat5 throughout the house for a few hundred bucks. He couldn't terminate it, because he's not licensed for low voltage, but he could run the wire itself and leave it coiled up in a single gang box in every room. The builder didn't think to offer that, but having a chat with the electrician when he was on site gave us that option.

All the <10 yr old spec houses in housing estate near me have got huge mould problems as somebody cut corners on either the installation of the insulation, the detailed design of the floor/ground connection or the ventilation, or all three. In the UK spec housing has a reputation for being very poorly built. Try claiming anything under a building warranty and you'll realise you get nothing more than minor cosmetic repairs without going up against the warranty company's lawyers.