I love my Martelli cutter; I could no longer use my Olfa because of the pressure it requires from the first finger. I found the youtube videos very helpful, showing how to clean and explaining the proper dynamic of cutting.
I have accidentally opened my cutter a couple of times by running it over the mat having forgotten it was closed. I prefer to do it with my finger, but you don't have to use the other hand, it is easy to put the shield back with the first finger of the holding hand.
I've never tried the 60mm, as the 45 works fine for me. I have found the blades to be nice and sharp. My cutter developed a glitch after about three months, so I sent it to them with a note, and received it back promptly with two replacement bolts and an explanation that my bolt had developed a groove because I was pressing too hard. It's back to working great with the replacement bolt.