I've got one, I mainly hack and do pleasure rides and I love it -and so does my horse, she moves much better in it than she ever has in a saddle with a tree. I've only popped over little logs and things, but manage those fine in it, I don't know what it would be like over bigger things.
I've often wondered though - do they have weight limits for these saddles? As I have a bad back and the only time it doesn't hurt is standing up in canter or bareback canter - which is rather slippy and I obviously cant jump bareback (FAR to scared to try!!). At 12stone would I squish it too much onto his back??
I am heavier than you (quite considerably!) and have had no problems - I have got the 'physio pad' which has additional support, and the new 'model' has additional padding in the actual saddle.
I have the "London" which is the dressage model and I love it. I school, hack and do small - 2'3 jumps in it and have had a few gallops and feel totally secure and comfortable.
I use it on Cairo, 16.3 clydesdale and Chancer who is a 15 hand gypsy cob.
Very comfortable and the saddle has never slipped or moved but I did buy the specialist pad for it. Cairo has never gone better and Chancer can grow away to his heart's content.
They do come in two sizes - I have the size 1 and am a size 12/14 and would not want to be any larger to fit nicely.
Takes a little getting use to, but everyone who has tried mine has commented how comfortable it was to ride in.
I may later invest in a show saddle for Chancer, but will keep to a treeless saddle as I really do like them and they suit my type of chaps.