I used one in a showhome once - funnily enough to show flexibility in a study. The only thing I would say is make sure that the wall you would fit it to is properly strengthened, and I would also have the suppliers to fit it, so that it is i. done properly from a safety point of view, and ii. you have one point of responsibility if you have any guarantee issues.
I don't have one but did consider what looked like a great idea from: www.studybed.co.uk as you don't have to keep taking things apart and rebuilding if you see what I mean. Not cheap though so may be of no help if you're on a tight budget.
I had one from somewhere years ago as a student in a very small accomodation. I could touch both side walls with my hand stretched in front of me and my leg behind. It was great for making more space on the rare occasion I had someone over for more than a shag and wanted the place to look less like a bedroom. The only thing I would warn it to make certain that you don't clutter the space in front of it too much, otherwise you'll have trouble clearing it for the bed. They come down and out farther than expected sometimes.
We've recently bought a house and are slowly furnishing it because we are far too skint to do it all at once I want to as many places to sleep as possible because we love having family and friends visit. We have a small study that I was planning on putting a single futon in (which may just be kept in the guest room until needed) as I didn't think a large futon frame would really fit.