I had one, hated it and gave it away!! It's difficult to inflate to the right pressure, doesn't sit easily under the saddle and the channel down the spine doesn't allow enough clearance if it's fully inflated.
Yeh, I have two. They are very good once they are fitted and used properly. I was in a large racing yard recently who swear by them.
Just be careful as the instructions that come with them are not very detailed so you will need to find a knowledgeable retailer or call Horseware directly
For advice.