It is very handy for weather, NOTAMs etc., but my experience of using it to file flight plans has not been that good.
The status messages you get back about the FPL's progress through the system aren't very clear, and neither of the ones I filed ever seemed to emerge at the other end.
I've gone back to the paper and fax method, but I'd be interested to hear if others have had more luck with Olivia, since it would be handy to be able file flight plans via the web using my mobile or PDA without even having to leave the lunch table.