If the planned development gets done it will be the same mixed use that was by all means successful for the Back Bay: residential, office, retail, hotel and a convention center. Major tenants have signed leases for new and current buildings (PwC, Goodwin Proctor, Battery Ventures) and the BRA appears to be dialing back its overemphasis on hotels (witness the sausage parcel 180). It doesn't seem fair to knock the Seaport in isolation, I think you need to at least include Fort Point. Fort Point is and will be to the Seaport what Comm. Ave was and is to the High Spine of Back Bay.
Couple the above with the fact that its on the water and has extremely easy access to north-south and east-west highways, the airport, and I don't know how one can claim that the neighborhood is DOA. I'd argue that there is now a bit of inevitability to its build out as a mixed use neighborhood that only started when Vertex signed an LOI with Fallon in 2011.
Picking apart an ugly building like Waterside is fair game (I agree its disappointing), but lets not forget the vaunted Back Bay has plenty of shit buildings (e.g., all of the Prudential Center buildings other than the Pru and 111 Huntington, the Colonnade Hotel, the Westin Copley, that mid-rise apartment building next to the mid-town motel) that are overlooked b/c, well, they have already been built and are literally and figuratively overshadowed by the gems.
I can't say I'm without bias, look at my handle for pete's sake. But when the build out is complete and the buildings aren't taken in isolation I think the general feeling about the Seaport will be positive and not a total loss like, say, the West End.
