Holy crap, I haven't seen one of those in years! They were somewhat popular among Pugsley owners when they (the Pugs) first came out. A friend had one and he always said it helped in soft snow. But they never really took off.

I only rode one once around a parking lot. It felt... different. I believe it was Bike magazine's review, but they nailed it:
Pros: it does exactly what it claims it does
Cons: it does exactly what it claims it does