I started with a Fiskers. Then found the Olfa (I have them in all sizes) - the straight ones, not the ergonomic ones. Love these. Have tried the Fons & Porter, not impressed. The Clover is a cheap one and so-so. Off brands are just that - off. Cutting surface, as everyone has stated prior, is key. You need a hard surface to cut on. (I use my dining room table when I need lots of space).Also, blades can and do make a difference. Change them often. I know some folks have bought blades at Harbor Freight and love them. I thought they were miserable as I could never get a sharp enough one.
One last thought - what is your cutting posture like? Do you sit or stand when you cut? If you are sitting, try standing up. I have a small cutting mat at my machine and have set my chair height so I can cut without too much strain or having to "push" too hard on the rotary cutter. Try different positions, this may also help.
Good luck!
Anita