Agree with pondhopper, the story has yet to be written.
My opinions formed as a rec. coach, one DD in a US Club select (not ECFC) team and another DD in rec., but looking to move into select this year, probably in US Club.
The "Seamless Soccer good:
They're pretty good at implementing the open player movement ideal. Good players are quickly moved up to the 'premier' Copa and Tango teams.
The coaching at those levels is very good, and very consistent.
They've 'trickled down' a ton of training and coaching development opportunities to rec.
The bad:
The coaching at mid level "regional select" teams is very inconsistent.
The team formation at the mid level is pretty weak. Some regions have several teams, others have none or one.
They weren't able to get the "community select" or super rec. team idea off the ground and have squashed attempts at creating it that I know of.
Overall, I'd say it's working great for rec. teams, okay for the premier level teams, but terrible for select. Ultimately the issues they're having with select are going to cause long term problems.
The basic problem is the "seamless" ideal ignores the reality of how and why kids choose to play on select teams and how they view development. There isn't a one size fits all pattern of development, and to impose that to the exclusion of other options is the crux of the problem. Eventually I think they'll change this, because there are a lot of intelligent and well meaning people at SYSA, but it is causing a lot of heart ache in the meanwhile.