I wasn't particularly impressed with any of the MI films; I enjoyed the opening titles on the first, but after that it strayed away from the series' roots and began to sound just like every other hollywood film, non descript, and very boring.

The second film was a disaster. Though normally a good composer, Hans Zimmer's attempts to make a heavy medal version of the theme was absolute torture. I spent the entire film wishing I either had ear plugs or a hammer.

Michael Giacchino's work on the third film was not only good, but brilliant. He revamped all of the music from the original series, not only the main theme, but the incidental pieces as well, re-working the instrumentation so that the parts that had sounded flat in the 60's now rang with the zing and beauty of a full fledged orchestra. The incidentals in the music were artfully tied to the action and emotion of the film.
Once again, Michael Giacchino has proven that, whatever the musical style may be, he is one of the best composers since the late Jerry Goldsmith.