Cute, but hardly useful for anything but the very basics. I simulated a 12 volt source through a 100 ohms resistor with a time step of .000001 seconds and it took over 4 minutes to complete.
The same simulation in LTSpice takes 4 seconds.
It's a lot more visually appealing than LTSpice but as far as functionality goes it's very limited. Possibly a useful tool for starting users that are intimidated by LTSpice, as it's widely known for having a sparse user interface without a lot of fluff. For students/teachers I think it has a lot of usefulness and basic hobbyist and tinkerer aspects it's passable for quick results on low resolution simulations, as far as being useful to a practicing engineer... not even close.
Whatever solving routines they're using are criminally inefficient due to it being web based. I could see this being useful if they'd turn it into a cloud computing application where the schematic was solved on native hardware at a server farm, and just the results were sent to the PC. As it stands the processing engine has no possibility of being practical. Though the schematic interface itself is quiet nice, that's about all it has going for it.
Great link though I think it fits the bill for curious people to start investigating electronics in a more serious manner than. I would recommend that anyone curious in electronics try it out, especially with the examples they give it's a great basic teaching tool.