I have owned two pair of Zu speakers, The Union, and the Omen Bookshelf.
Both were very good for the money. The Union came close to my Vandersteen 3a sigs, but lacked the finesse.
Newer models are probably going to provide better sound reproduction, so choose wisely if you are buying used.
That being said, I think they are quite a bargain for someone on a limited budget. Of course, every manufacturer is going to tout some technical attribute of their product, so take everything you read with a grain of salt.