I had a circular cutter fitted three years ago, when we had a new engine, shaft, prop etc. I does work, as I ran over a mass of old fishing net off of Gilkicker last year. However, the net still got wound round the prop, but the mass was cut away. I motored in, at about 2 kn max. and had to cut the rest away with the trusty kitchen knife once back on my mooring. With a 14inch prop, I finished up with about a square yard of net, with neatly cut edges wound round it.
They are not too expensive and probably worth the investment. However, if they do their job properly, you would never know.
I also have a proprotector fitted and it works against lobster pot lines (at least two this year in poor light) but doesn't do much to a tesco's plastic bag. However it was all that could be fitted in the space available.
I fitted a Stripper 2 years ago, but only because I picked up a brand new unused one from the internet for £100!!, about a third the current price. Don't know whether it's worked as I don't think I've run over any ropes. Certainly gives piece of mind and it came top of the survey.
Go for it.