Marvel's first foray into independent production of animation is a mixed bag, loosely interpreting the framework from Hitch and Millar's mad "Ultimates" storyline but not retaining any of the mean spirited glee that made the work so entertaining.
Let's start with what's right: the animation is solid, and the character images closely mirror Bryan Hitch's ideas (while admittedly not approaching his level of detail). The voice acting for some characters (Henry Pym, Dr. Bruce Banner, Janet Van Dyne and especially Olivia D'Abo as the Black Widow) was spot on with great casting choices and inspired line readings. There were a number of legitimate surprises (I liked almost everything involving Giant Man, including his clear lack of experience).
It's not all wonder and glory at the Triskelion (not sure that it was ever called that, but it looked the same, so whatever). This is definitely a film for fans, as the opening sections has so much information glazed over it was like watching a cartoon in a Krispy Kreme. Also, it'll be very quick to notice that this is "not" the Ultimate version of Captain America, but his much more whiny and navel-gazing Marvel Universe Earth 616 counterpart. Second, you're also getting an Iron Man much closer to the 616 than the Ultimate Universe, in that his billionaire playboy shtick is much thinner and much less extreme (overall the sexual energy is muted to virtually nothing, outside of flashes of Pym's jealousy). Third, Thor is virtually a guest star, appearing in so little of the film that he barely even mattered. With those three elements in hand, that means you can't have any of the interplay between those characters that stood at the core of characterization in the original work.
There are other issues -- voice casting snafus, with Andre Ware as Nic Fury was lackluster at best, Justin Gross providing a whiny early Luke Skywalker-esque Captain America and Marc Worden's Tony Stark barely even showed up for work.
But perhaps judging it against the source material is unfair, so let's look at the material as it stands. The Nazis are helped by aliens for reasons that are never actually explored. Right. Somehow they lose WWII to the Allies anyway. A bit farfetched, but let's go with that. The aliens stick around to buzz nuclear plants and generally being annoying -- again for no discernible reason. Ooookay ... mysterious powers that be demand results, and a rag tag team of malcontents and misfits has to come together to win the Little League championship, er, fight off the aliens. Sort of. There's a crazy Dr. Jekyll and Mister Hyde subplot with Banner that just serves as a distraction and reason to extend the final fight scene. Things just somehow failed to make sense, with a conclusion that's anticlimactic and the "Ultimate Avengers" kind of getting their butts kicked through most of the movie. Largely due to their own incompetence. Which makes Captain America whine. Blah.
Tossing back on a continuity cap, the sequel is practically built up with one word -- "vibranium" -- that leaps out to Marvel fans. The action sequences are entertaining and it packs as much punch as your average Hollywood special effects blockbuster (at a fraction of the cost to produce), but if you're not a Marvel fan or a young child not worried about people being disintegrated to death on screen (in the age of "Grand Theft Auto," who even bothers anymore?) it's not a very gripping story. A mild recommendation for people into superhero smashing.