If it helps, I was actually looking at the Essex EUP123 in the Jacob Bros. store in Philadelphia. They're primarily a Steinway family dealer, so I'm not sure how accurate their promotions were, but I thought it was worth noting that they were selling the EUP123 for 5,990$--and they were shocked and appalled to hear that someone might be selling one somewhere for less. (Apparently those steinway family pianos are supposed to be marked on a fixed price across the country).
I actually thought the essex compared fairly well to the same sized Boston, and at least by their prices it seemed to be a good deal. I also looked at it in comparison with the U1 which you've already noted was too expensive, and while I preferred the U1 for dynamic range, voicing, and attack, I think the Essex seemed better suited in some ways for classical music. According to the dealer at least, the new line of essex is a significant improvement over the prototype so maybe that's where some of the disagreement here is coming from. I was actually deadset on DISLIKING essex as much as possible going into the store, and was reasonably impressed by what I saw. I'd think it would be a fine piano for a serious thirteen year old. My parents bought me a baby grand about that age when I was starting to seriously think about a career in music, and while it was really a pretty mediocre one, being able to choose the piano that most impressed me made a big difference when it came to practicing. (That may be another reason to get an Essex. . . It has a pretty mellow tone, and if you're going to have a kid playing more than several hours a day, a brighter piano like the Yamaha U1 might well drive you bonkers after a while. If he's interested in popular music rather than classical however, I'd take the advice of others on the post and look at the other Yamaha pianos seriously).
If anyone out there actually owns an Essex, however, I'd like to hear from them too because I'm considering it myself. II do know that in comparing the Boston and Essex models to Kawai and Young Chang or Pearl River pianos, they are VERY different instruments. I didn't like the Pearl River pianos at all, even the new ones which are supposed to be such an improvement over the ones that Larry Fine disses.