Pretty happy Silvrback user here, but I do kind of miss the "quick idea" interface from svbtle (wp version in my case).
How well has Silvrback held up under heavy load?
Silvrback performs well under stress. It's setup to autoscale when a traffic surge is detected so the platform can easily handle articles making the front page of HN. Unicorn and memcached also really help.
It's similar to svbtle in its simplicity.