I think that I, along with lots of other fans, were so excited and amazed by FOABP that when TI came out, it was just way too hard for it to top FOABP. Expectations were high, there was a new bar to reach and I think TI just comes out just under that high-set bar.
However, that doesn't mean it's HORRIBLE album on it's own right. It was just over-shadowed by what came before it. I'd say it's as good as Deadwing (in many spots), but I also think the explicit concept of the main album probably turned some fans off from the onset. But to be honest, it's not much different than the ideas that their previous 3 albums were conceptual in nature as well, if not more so than TI in terms of creating a story.
TI isn't bad, and I do enjoy it from time to time, but it doesn't really do much different than the previous 3 albums did, it just sort of takes what they did, both good and bad, and refined it a bit more. I do like the idea and concept of the main disc, and I really enjoy the other four songs on the 2nd disc, so it's not a bad over-all.
I still like IA, DW and FOABP better though.
-Marc.