I just received and tried Mastmate. It was fairly easy to finish (with own slides) and set up (pull up the mast). I tried to climb once without anything connecting me to the mast - and that was pretty unpleasant
So I furnished a kind of chest level loop/shoulder harness, clipped it around the mast and things went much better. Easily got to the spreaders and was able to finally fix lose tape on a spreader boot. It is actually possible to get to spreader ends (with one hand at least) from the ladder, if the chest loop around the mast is sized right. Going up from the spreaders required two loops (to connect one above, then disconnect the lower one). In any case, when my feet were just above spreaders - I figured I don't like it that high just yet
So I went down for the day. I'll try again next week.
Bottom line - the device is easy to use and got me much higher than anything else did, just as long as I feel comfortable with it. So, a plus for mastmate.