I am sure most of their tanks are really good. but some are bad. on one thread I saw on another forum, a poster had just purchased a 20g nano reef tank from them (done rimless) and the silicone work was awful. they posted pics and the other members of that board said it looked "like a 5 year old had done it"
obviously, this person was upset, called to ask about a refund/replacement etc. and was told off. he was pissed.
as mentioned, it seems that a lot of their negative press seems to be from custom orders. if you don't get exactly what you want, and you call them about it, they apparently don't help you. at least some of the time. those are the times everyone hears about.
and that pisses people off. after all, they paid for a tank made to their specs and when its not done right or done properly (sloppy) they get mad when they are not helped afterwards. thats probably why there are so many negative threads all over the fish forums about them..