Yes I've heard about them and come across customers that have used them to good effect and results. Generally they provide a good service and help cut out the uncertainty of trying to find a good, trusted supplier and at a reasonable cost. You are quite right in that they are totally funded by the companies they provide the contracts to and in that respect peace of mind for the customer and it costs you nothing for that. The service isn't for everyone as some people clearly want to control things themselves and to be fair, most people are capable of working out what is a good deal anyway providing they have the time to spend on the process and do their research well such as looking at this forum for pointers. However, for those who simply want one visit to their home then it's a good idea.

My only tongue in cheek thoughts are that (a) not every conservatory company would want to fit into the NCAS way or even sign up with them and a lot of those will be very good companies indeed. Therefore, the choice open to you both on technology and several other things may be a bit narrower than otherwise (b) we're all human and their own reps must have personal preferences and may themselves be only bothered to consider a few choices for you. Again I'm sure they'll be good choices but there would be a narrowing of the field again. (c) it's a leap of faith in that you have to believe they'll always act in your best interests. On the one hand there is no reason to believe this wont be the case and there wont be any bias on their part but on the other they have to make money to survive themselves and pay their people and so a larger "commission" or kickback from x and y company say may just sway them in a certain direction on the eventual choice. (d) If you're a good conservatory company and your people (sales people to boot) understand the products thoroughly then I question how a third party can be that well immersed in the technology and advantages between x and y company offerings. Unless they've gone into each of those conservatory companies and understood how the thing is made and the techniques used and even the ethos of a company then I do wonder how you can recommend or advise from a somewhat "remote" - albeit informed - position.
Overall, you're under no obligation anyway so give them a twirl. At least they're sifting out the "bad guys" and protecting your deposit with the way they vet companies so why not!
Generally I think they're a good idea for those customers who want that sort of approach and they do offer a pretty good level of independance in an industry that has its fair share of dodgy sales approaches.
I actually recommend reading their web page that covers "Hall of Shame" where they go through the various sales techniques used by companies.
General site is: http://www.nrwas.org/html/how_we_help_you.html

Kevin
Kevin Poole Glenhampton Conservatories
www.glenhamptonconservatories.co.uk