I have had a Fiskers, Olfa, a Martelli and a Gingher - they're like fishing lures... you never know which one you will like best!
Anyway I always go back to the 45mm Olfa with snap back retractable blade. I always end up leaving my blade locked open and with the Olfa - when you relase the handle the blade snaps in. It's the only one we carry at my LQS and recomend it to new quilters.