I'm using Sauce Labs primarily for manual cross-device and cross-platform testing.

It allows me to have a wide array of combinations at my disposal almost immediately, which is great (maintaining a suite of browsers myself is a real pain) and it's really speeded up our testing workflow as a result.

I will say that the VMs are pretty slow at times - there's lag on the mouse movements, which can be annoying, but it's fairly consistent and never really gets in the way.

The ability to have multiple sessions open in different combinations at once is a real boon (though it's a shame they're not real browser tabs as they'd be easier to manage).

Overall highly recommended, and eventually I'll be using it for automated testing too (right now our app is testable with mock HTTP simulation at the Ring adapter level).