Edit: I have found it to be much more reliable in Chrome than Firefox. I haven't tested much on other browsers.
It's relatively laggy, at least for the sorts of game I like to make. It does what it claims to, but you've got to understand that you dont have the capabilties that you do making your typical standalone executable. Additionally, it introduces a lot of freeze bugs that I've been unable to fully contain in my more complicated projects.
That said, the jump to being able to drop things into a browser is wonderful. Whether it's worth the steep price is up to you, I'm poor so I kinda regret it, but if you have money you probably wont.
BOTTOM LINE: It's expensive, buggy, and kinda slow, but it still works, and that can still be worth it to the right person.