Pros: works as a decent amp, and the bass boost feature is surprisingly effective
Cons: the target market is a bit hard to define
That's my whole review. In summary:
The zo is an interesting product. It seems like it would be just marketing fluff, but it actually does work like they claim. I was able to get (seemingly) hard hitting bass out of some headphones that had never been able to produce anything like that before. As a portable amp, the zo is a bit better than most cheaper amps you would find, such as the various cmoy based designs on ebay. But the real star of this show is the smartvector bass contour EQ feature.
My only issue with it is that the target market is a bit ambiguous. It seems like it would be an entry level product for beginners, but that's decieving.... most really cheap consumer headphones are already too bass heavy so this won't pair well. If you have a decent budget headphone or IEM, it might arguably be better to spend the $100 on an upgraded model rather than the zo. So it ends up being enthusiasts who already have several headphone models who will get the most out of the zo; but those are the same people who are likely the most skeptical of this this type of product. At least digiZoid offers a free trial period.
I'd like to see this technology licensed to some higher end equipment. This could be one of those things like crossfeed that some people would get a lot of use out of.