If there was any doubt that Ultimate Captain America is a different version of the character than his Marvel Universe counterpart, this miniseries should lay them to rest. Whether such differences mean that the character isn't really Cap anymore is debatable, but certainly, when the story is this smart, it's easy to dismiss any worries.
Ultimate Steve Rogers spends the concluding issue of his first ever miniseries locked in combat with the Ultimate version of Nuke, the "other" super-soldier. Although issue-long fight scenes can often feel too brief and inconsequential to be read as a single entity (rather than in their intended place as the big conclusion to a trade paperback), Aaron and Garney manage to keep the sequence interesting and engaging throughout. It's brutal and graphic, and while that might not match up to your idea of Captain America, it is, in many ways, a natural extension to Millar's reimagining of the character.
Garney's art is perfectly cast against Aaron's script, rendering every shocking moment with the artist's expert storytelling. It isn't until the final sequence, however, that Garney and Aaron's mutual strength becomes apparent. Hawkeye and Cap's sober discussion places the previous issue in an interesting context, and offers a surprising final note on which the reader's thoughts can linger, due in part to Garney's subtle depiction of the characters
If anything's a problem with the series, it is the fact that the violence and one-liners are a bit over-the-top, even for Ultimate Cap. It's possible that Aaron is trying to deliberately make the violence shocking to highlight how odd it is for us to cheer it on, despite the context, but it's equally possible that Aaron (or his readers) might simply be missing the point.
As a result, "Ultimate Captain America" feels a little like it's trying to have its cake and eat it. Particularly for a line that, notionally, is supposed to appeal to a wider audience, it's hard to say whether this is the sort of comic you'd expect to see with the Ultimate name slapped on the front, both in terms of its subject matter and the execution of the same. The idea of someone going from the Captain America movie to this isn't one I'd necessarily be comfortable with, were I running Marvel.
Even so, the series itself is well-constructed, challenging, and interesting, and all the other things you'd expect from Jason Aaron and Ron Garney. It's one of the better things that the Ultimate Universe has produced of late (although that's not saying much) and if you want to read a Cap story that's completely different from any other, this is undoubtedly a prime candidate.