Fleur I use one and it's pretty good... you do have to be aware though that they have a failure rate of 10-15%, but if you are using it in conjunction with NFP and possibly w/d at fertile times you should be ok . It doesn't really kill the mood as such cause it only takes ten seconds to put in - not much of a mood if that can kill it! LOL
Family Planning is the best place to get them fitted. Apparently it is quite hard to get hold of them these days but if you can get fitted for the right size you can order it on the net. Feel free to PM if you have any questions.