While the emotional part of the plot is satisfying, and the action scenes -- accompanied by the usual cranked-up multi-track music and effects -- are exciting, there is still the issue of plot mechanics. Even more than its predecessor, M:I-2's story relies on many implausible motivations and cheap sci-fi gimmicks. While you can give Towne points for at least trying to explain most of the implausibilities rather than simply assuming that no one will care, his explanations aren't very convincing. On a moment's reflection, you realize the reasons that Sean and Hunt do certain things either make no sense or are so artificially contrived as to irritate.
The film also indulges in the first film's worst device -- the perfect masks that were an excuse for the cheapest sort of identity reversal. "I look like X, but really" -- rrrrrippppp -- "I'm Y!" While treading delicately to avoid spoiling a surprise that won't in all likelihood surprise anyone, let us simply say that the final use of this nonsensical gimmick completely defies logic, even if you suspend disbelief long enough to accept the gimmick itself.
Be forewarned: Abandon all hopes of common sense, and enter the theater with high expectations for visceral entertainment. You won't be disappointed.