So I tried the guide with scissors and with the rotary cutter and it worked pretty well. End of review. Hah! No, it was good but didn't quite have the wow factor I'd half hoped for. Which is totally unfair to the product of course, because it wasn't advertised as promising to increase the wow factor in my life!
The seam allowances it produced were nice and fairly even, although I found that starting off at the edges could be a bit hit-and-miss because the guide doesn't line up with anything for almost one blade-length. I ended up making a small mark to line up with at the start which helped. You can also only go one direction around the pattern piece, which I'm not used to as I tend to cut whichever way is easiest to reach. I had to move myself around the cutting mat which got a bit tricky in places! It was even harder starting off at the first edge with the rotary cutter, but I actually found it easier to be accurate with it, as the scissors tend to skew a bit as you close them and things just looked a bit rougher. The rotary blade seemed to really lend itself to that kind of cutting with a nice smooth action. I found I had to close one eye to reduce the depth perception so I could keep the guide lined up over the pattern edge as it's sitting up a lot higher with a 45mm wheel than it is with a pair of scissors. I probably looked like some sad one-eyed pirate wanna-be while cutting!
I'd say that yes it probably saved me a fair bit of time not having to add on the seam allowances but it wouldn't be as accurate as cutting around a pattern with them added. I definitely wouldn't use it for anything that required super-accuracy (like quilting) or with lots of ins-and-outs and fussy little nips and tucks. It's best saved for simple shapes. It also has limitations on how much of a seam allowance you can add, I think just under 3cm is the maximum. It's not like you have to add more than that very often though. Ummm...oh and the only cutting in one direction thing - but that's more a personal preference for me, I like to
Bottom line? Considering how inexpensive they are they probably do deserve a place in your sewing kit, especially if you sew frequently from Ottobre, Burda or the Japanese books.