I think because of how many ups and downs it has had, Ultimate Fantastic Four is still the least coherent Ultimate series... It's getting a hell of a lot better now, and I think Carey's going to take the book in good directions, but it still doesn't have the same punch as a good USP comic.
I always read Ultimate Spidey first on my pile because every month I hope it gets better, and recently its been picking up again (I really liked 100, and am digging how different from 616 Bendis is taking this... Especially bringing the CIA into matters).
People can point to continuity inconsistencies, weak writing at points, poor characterization at others, but its still one of the better books on the comic book racks today... And now that DC's golden streak is running off, and Marvel's about to peak and descend again with all this Civil War stuff, the only books that I know I won't be dropping are the Ultimate Comics, and especially Ultimate Spider-Man.
With Millar leaving Ultimates, an uneasy story-quality on Ultimate X-Men, and a new storyline beginning in Ultimate Fantastic Four, Ultimate Spidey is still going to be on my pull list in six months because Bendis has made me care more about Peter Parker than any other Ultimate Character. That's the strength of a good Bendis Book, and that's why I'm not giving up any time soon.
I revise my answer to 4.5 upon consideration. Its not perfect. But its still a damn good comic.