I Love my Q-snaps!!! I have 3 sizes...4x4, 8x8, and 12x12 8x8 is my favorite, I rarely use the small one, and use the 12x12 for beading.
I must admit I have an ulterior motive to this question also. My neighbor is quite handy and is going to make the home made ones and start selling them at the local flea market and we are looking into etsy and ebay. We are even brainstorming pvc/qsnap type scrolling frame and lap frames. So we are trying to find out the most popular sizes to make a few of first and see how it goes. Of course they would be cheaper than the qsnaps :)
I love the 8x8 and 11x11. I also like 11x14 for bigger projects so I don't have to move them around. I don't have the 6x6 size so I still have to use a hoop for smaller projects.
I have the 6x6 and have only used it once. I've had it about 4 months. I prefer a hoop for somethng that small its easier on my hands. I also have the 11x11 which I am using with my 6 Fatmen SAL. I would love to have the 11x14. I recently bought a scroll frame (The Handy I think) with the pvc clamps. I like it a lot except its hard to put the clamp on and have my fabric straight when attaching the clamp.