Adium is good if you are looking for the core features of MSN, and wish to use other IM services at the same time. Adium is indeed the multitasking king of IM programs. However, if you are interested only in a replacement for MSN, Adium may not cut it, for its lack of audio/visual components, and numerous MSN-specific features.
Mercury Messenger (Mercury - News
) is a closer alternative in terms of feature set, but its closed source, and written entirely in Java as far as I know, making it painfully slow at times.
aMSN is another alternative (Alvaro's Messenger
), which I haven't used in a few years. After looking at their website, they seem to have made some nice improvements, and it could be a fairly decent alternative. However, in the past I did find it a tad quirky, and crash-prone. Again, that was a few years ago.