I seem to be a bit late to the party, but I did play with SystemVision for a bit and I have some feedback.
Drawing a schematic is actually pretty smooth. I find it somewhat less clunky than CircuitLab.
The error message are much more helpful than they are in Circuit Lab.
The abstract blocks could be very helpful for system-level questions. Circuit Lab doesn't have PID blocks or three-phase transforms.
Being able to put a waveform directly on the schematic is nice.
I don't see any way to have both a reference designator and a part number. This is a show-stopper. Fixing this could be as simple as adding a second text label. (Per Olin's post, the default part number should be blank.)
The waveform viewer doesn't update automatically when I rerun the simulation. This is confusing. There should be something to indicate that a waveform is old.
The "ideal" op amp defaults to a gain-bandwidth product of 1 MHz and an output resistance of 100 ohms. This could confuse college students trying to simulate their homework.
Clicking and dragging to move the schematic around is extremely laggy. So is zooming. (I have a 3.5 GHz Core i7 3770k with 16 gigs of RAM, so I'm pretty sure it's not me.) This is using Firefox with three tabs open.
I don't see a way to save a waveform as an image. I suppose I can just take a screenshot, so I'll call this a nice-to-have feature.
Overall, I like it and I think it would be a good addition to the site. I'd probably use it more than Circuit Lab if the part number issue (and lack of text annotation) were fixed.