To my knowledge, the Roof is the only helmet that doesn't carry the recommendation NOT to be word open; all others suggest that you should not ride with the helmet up. However, you often see people on Goldwings, K1200LTs, and the like, wearing theirs flipped up because they are protected by their large screens. But, the manufacturers would still tell you that they are not impact tested or designed to protect you in a fall without the chin bar fixed in place.
There have been a number of comments on the mailing list and in other threads about the Roof and the jury's out. There are a lot of negative comments, particularly about noise and comfort.
I've owned/possess three flip ups: a Vemar; a Shoei Synchrotec; and a Schuberth Concept. All are noisy. The Shoei is an excellent helmet, very comfortable and the least noisy for me. However, I like the Schuberth best. Like the BMW System 4 Evo, the visibility is superb, very good peripherally. Comfort is excellent. The Schuberth has a sun visor inside the visor. The BMW can be converted into a Jet style helmet or a motor-cross style helmet. My girlfriend has the BMW System 4 and in a small size (54) finds it quite heavy (because the shells are basically all the same size). At first it gave her neck ache. I have a size 61 Schuberth and find the weight pretty good compared to others.
Visor removal on both are excellent/easy. Ventilation on both is good, but the Schuberth has a neat visor mechanism that lets you pull the visor forwards to create an airflow underneath. Unfortunately it means that the seal isn't as good in the rain.
There are some very cheap flip-ups on the market now, but basically they'll wear out quicker inside (like my Vemar) and not feel as comfortable on your head.
The Shoei is the most expensive at £300 and I don't think it's worth the extra compared to Schuberth or BMW. Don't bother with the BMW Carbon. It actually feels unbalanced because the chinbar isn't Carbon (that's what I found anyway).
In summary . . . you'll find all flip-ups heavier and noisier than full good face helmets. In some cases you'll find full faces more comfortable. The advantage of the flip-up is the flip-up (going into petrol stations, having a fag or a bite to eat, snogging your partner - all impossible with a full face). The BMW and Schuberth feel very bright and airy with great visibility. The BMW can be converted to an open face. Personally I'd avoid the Roof unless you want it just for its radical looks.
Simon E.