So far I've had no issues with mine. I did manage to crack one, but I can hardly blame that on the tank. We'll have to see how they hold up over time, but right now they are by far my preferred delivery device.
I've never tried a vivi nova. So, I have no input as to weather or not they are good or bad. But, in the short time I've been on the forums I've seen a lot of threads about leaking and wicking issues. I've also seen a lot of posts by people that think they are the best thing ever created. Like the Protanks, I have a feeling it's just the by-product of being mass produced and highly popular.
To each their own I guess.