Say it takes me 10 actions to fully test one feature and all its cases. Spread that across FireFox 3 + 3.5, Safari, and IE 6 + 7 + 8, that would take a total of 60 actions for one feature. With a web app that has 20 features testing it once a week I would have to pay $326. What if a test fails, do I have to pay again?
I don't see how anybody can use this with such a hefty price tag.
Most of my automated tests run in about 5 seconds or less. With EC2 prices at about $0.10 per hour, I can't imagine ever using a service with such a high premium. Not only because the price is nominally expensive, but also because it's a commodity service that's being priced at a HUGE premium.
For me, I'd consider the service if I could offload my grid testing to the cloud for around $20 / month.
That way I could run a bunch of smoke tests after every push to make sure nothing broke and a larger set before commits. But that can get to be a lot of tests especially when you re-run for failures.