I was just here last weekend. It's pretty cool, but I also think people are kind of overrating it based on its current incarnation.
Many posters here use this as an example to rail against the Seaport. However, Assembly Square is still anchored by a strip mall, which is something we will hopefully never see on the Seaport. Also, Seaport land is more valuable, so it makes sense to see more of the large blocks over there, whereas Assembly can get away with 5 story wood structures to increase density.
Once the current set of Seaport buildings are done, I would take that over Assembly in a heartbeat. It doesn't have the luxury of being beside a shopping center, but is overall denser with more futuristic architecture. I do like Assembly quite a bit, but all the hoopla is really centered around one intersection. It has some good draws, like the Lego Museum and movie theater, but it had most of those things to begin with. (not to mention Good Time Emporium, the single best/affordable place for casual recreation around here, may it RIP)
I have been to the Seaport in the last few weeks as well, and at this point the cumulative amount of buildup over the last few years puts it ahead of Assembly for me. I'd rather walk around there any day.