I’ve used Shadow in the past but not with Blender. I’m planning on adding it to the comparison thread @blenderrocket posted above but their subscription model makes it harder to work out their true price to performance values (since it depends how much you use them per month).
One thing I do remember from the last time I used their service was that it’s difficult to just leave a machine running when disconnected. If you want to render a job but don’t want to have to stay connected to the machine your kind of stuck though that may have changed.