I found it useful and pretty but I did a lot of my charting on paper and found it just as good.
I'd record temps in a notebook each morning and then transfer it to my paper chart that I got from somewhere on the web. It was just as easy to track and for me it was fairly obvious when I ovulated. (Plus I was using OPKs).
Personally I don't think I would have coughed up the $ for the subscription - after all you have lots of lovely baby stuff to save for