I'm adding my 2¢ because I just "discovered" iTunes Match yesterday and decided to subscribe thinking there was no downside (doh!).
Well, firstly, there's the enormous downside of Apple's structure not being able to keep up with (what they should have known would be) enormous interest... my iTunes has been matching for >20-hours and has yet to reach half-way up my library. BTW: I thought I read the iTunes Match info fairly thoroughly prior to subscribing and did not see anything about it being for songs vrs full library.
And secondly, and even more important to me, is that Apple's method of determining duplicate songs is not robust enough, especially for classical music and audio books (e.g., LoTR). I have >12000 "songs" in my library, and Match has determined a boatload are duplicates. They are not of course and even if Apple's duplicate-determining algorithm just looked at disc & track numbers it would "know" that. Anyway, there's a fairly good chance I will not have one complete album upload/match out of >500 albums. That's nae Scottish, in anybody's book.
Gotta say, using Garage Band to individually alter >2000 "songs" on the off-chance that may assist Apple's duplicate algorithm to be correct is not gonna happen. If, after the matching/uploading is complete, I still have >even 100-not-duplicate-songs listed as duplicates, I'm getting my $29.95 subscription back.
... and C: does anybody know why Apple's Lion spell check doesn't work on Apple's discussion forums???