I used it for a year or so but I did not renew my subscription. I think the concept of it is very good, but it needs further development to become really useful.
If you are already familiar with Flitestar, you will find the interface familiar too. But it is much simplified in comparison to Flitestar and a lot of advanced features are missing.
It's speed is dependent on your internet connection - it is just about tolerable if you are on a fast internet connection - but forget it if you are using it (say) on GSM Edge or GPRS.
You have to install a small program on the local computer - which is a big drawback, as you can't usually do that in say an airport ops room. It would be much more useful if it were a purely remote application that could be logged into from any internet browser. Also, there is no access to Jeppview approach cards - so you can't make a full trip kit. It would be a big improvement if it were possible to download "Pay as You go" approach cards.
There are some small bugs - just like Flitestar the automatically generated ICAO flightplans are pretty ropey and require editing before submission - but annoyingly JIFP does not remember your changes, so you have to re-edit the flightplan every time you load a stored route, or even browse away from the flightplan page for a moment.
You can store aircraft models on the server (with similar interface to Flitestar) but they are a little simplified. You can't store Corporate Flitestar models.
In summary it is not anywhere near complete or developed enough if you are corporate flying. Even for a GA pilot, the lack of approach plates greatly limits its usefulness.