I bought a showhouse five years ago. I bought it for two reasons. Firstly, as a first time buyer, who had only the price of the house, I thought it was good to get everything up front. Secondly, it was the only house left of it's model type and I liked the layout. The following are just a few pro's and con's that I came across:
1. Expect to replace the carpets/flooring within 12 months. Showhouses are only sold at the end of the development and so a few hundred people have marched through the house to view it. The carpet in my hall in particular was fairly threadbare, but I still have the same carpets upstairs.
2. I lilked the fact that I coukd just move on with my few bits and pieces and that was it. Things like light fittings and blinds all add up when the budget is tight. I know that I had paid for them all ready but at least it was all taken care of before I moved in. The good thing is that most of these houses as you will have probaly seen are decorated in a fairly neutral style. I still have most of the furniture that came with the house five years on.
3. The best thing for me was that the garden was not only laid down with grass but half of ot was planted with fantastic plants that I would not have had the money or knowledge to put in myself and since its established, it takes very little to maintain.
4. I have had no problems with the building itself, but as one of the other posters said, I think that it depends on the builder. Make sure you get a good snagger though.
Hope this is of some help. As regards the cost (I am useless at percentages!), I paid IR£180,000 for the house and the same "bare" house of the same model in the estate sold for £170,000.