I had one -- it never was very comfortable, it slipped, and I never could achieve the right height. You can control slippage somewhat by using a non-slip material between the cushion and your trouser leg, but I found this wasn't the best solution. Plus it was a pain to carry around even with the bag that it comes with.
Switched to an Ergoplay, and then a DeOro. Love the latter: small, easy to transport, highly adjustable, and it stays in place. Worth a look.
2015 Connor spruce/Indian rosewood
1978 Ramirez 1a cedar