Fiskars is my favorite circle cutter. I use the USX with templates, on a regular craft mat. I haven't tried a glass mat yet. What I like about the Fiskars templates is that you can see exactly what you are cutting. It's easy to precisily cut a circle in a photo or on patterned paper in exactly the place you want to cut. Just place the template and cut with the USX. Some people complain that the USX does not work, but the trick is to have the template in the right direction, with the Fiskars name facing up.
I also have the Creative Memories system, but I no longer use it. Their template is great for exact positioning, however, you have to move the template to place the circle down for cutting. Doing that is harder than it sounds, unless you are not being precise.
LOVE my cutterbee set - but glass matt is a must with it. I have a fiskars shape cutting set - HATED it. Even with the glass matt it was still a pain to use and hard (for me) to get a good clean circle without stuff sliding all over the place.
My cricut expression could do the same too but sometimes the low tech solution is easier to pull out and use (finding a centerpoint with the cutterbee seems to be easier for me than the doing it on the expression and I can visually see what size I'm going to cut too)
I have the Creative Memories circle cutters and love them. I just bought the new 12" circle but it hasn't come in yet. I use a self healing mat with my cutters but not the CM mat.
I also have the Fiskars cutting system. It works very well once you know how to use it. It isn't the easiest to use at first but I do agree with Luvmyfam above that you can see exactly what you are cutting.
I would definitely recommend the CM cutters if someone was buying their first set.