We have bought 2 brand new properties.
The first one was finished before we bought it, so we could see what we were getting. The second one was bought off plan. So I had no idea what it would look like inside. DH had seen round the shell of the other one similar but I had to rely on his views!!!
One thing I would say is that developers can be really pushy with the exchange date. When you commit to buy they normally specify the exchange date they want then and they will hassle you to get it.
This meant that with our second property we ended up exchanging 6 weeks before the house was even finished and living in a hotel for 6 weeks, so that we didn't hold up the sale of our property.
You can normally though have a say in all the tiles, kitchen units, paint colours etc which is nice. Also if they are still building around you, it is always handy to get to know the guys working on site. That way if you have any little jobs you need doing, such as extra patio stones to be laid, a path to be done etc, they often agree to doing them for cash and for a far cheaper price than you would pay normally. The site manager is always handy to know as if you have any issues with the house, you can get them sorted out without going through customer services (which is often a blessing in disguise).
You will have a constant battle with dust though if they are building around you. We used to have a fine layer of red dust on most of the window sills and they agreed to clean our windows once they had finished building near us.
Also check what you get for your money. We managed to get carpets and all floor coverings included with our second house but not with the first. So that was an extra expense. Also all we got was dirt and a few bushes in the garden, so we had to pay for turf etc. Normally as well, unless you buy the ex show home. you will have to get curtain poles, curtains and blinds etc for every window as they are not normally included.