I got this at launch and still use it regularly.
Its too buggy to leave on all the time, and I don't want the windows all the time. But turning it off/on takes all of 10 seconds, so its hardly a big deal to do so when needed.
I use it to run various streaming videos while I surf the web and that is critical function for me. Great improvement, and you can use 100% of the screen space effectively if you set the windows up right. I also use it to keep an app running in the background if it won't by default behavior, sometimes I leave videos going in the back for the audio but also apps that have a 10 minute time out, something downloading, or just something I don't want to lose its exact state.
The update that allowed apps to open in fullscreen improved it a lot, doing that manually was very inconvenient and caused more crashes I think.
So yeah I leave it off 90% of the time, but the other 10% is well worth the purchase.