hI meliesmummy, I am using it at the moment, this is my first month of trying it out. I am a part of a study comparing the fertility duo with babycomp and using opk's. I have to say my initial impression was that I liked the convience of not having to wake at a particular time to take my temp, but when I started using the babycomp I liked this too as although I had to wake to put it the termometer in my mouth, I could go straight back to sleep and not even have to open my eyes! OPK's I don't do well with, so not my thing.
I have been charting including temping for about 9 years now, so am enjoying trying out these new devices.
As well as the not having to wake to take temps, it is great to actually see in advance when your peak fertility time is coming. I was shocked that despite knowing my cycle very well, this system accurately predicted my peak fertility time days in advance!!!!!! I don't know how it works but it so far seems to do really well. And I also have to say the customer support from fertility duo is excellent.
I think that although it is expensive, they do have a money back guarentee thing on it, which I find impressive, so I would say that any long-term ttc'ers who were getting to the assisted conception / or fertilty investigations stage, it would be a reasonable investment (compared to fertility specialist prices when done privately!).
As I say, having just started using it, I am so far impressed, but perhaps ask me again in a couple of months time, and I will be able to give a fairer answer!