Overall iTunes Match has been worth it. There are some very interesting anomalies.
In some cases Match fails to match songs on an album for inexplicable reasons. Several albums of early 80's genres were matched for all but one song. Though the album had been ripped and encoded in a single shot something prevented these songs from being matched.
An even more interesting case is this album of Franz Danzi Concertos:
Note that Match found 6 of the 7 tracks and uploaded just one. However this album is not available in the US iTunes Music Store (not sure if it's available elsewhere). The third track is available as a part of another collection but the other tracks are not available. Yet with Match I can (and have on another computer) update these tracks to a better encoded version of the tracks.
Another interesting bit is that Match will not overwrite your metadata. If you have a song title, artist etc incorrect it won't change that. It will, however, match the song with the correct song from the correct artist.
One of the bigger disappointments was some iTunes Originals albums and some Pre-order bonus tracks that are not available any longer in the store and though similar versions of some tracks are available they aren't eligible for upgrade where the same track. While disappointing it is to be expected and there don't seem to be many options for licensing this content otherwise.
Finally when iTunes Match detects duplicates it seems to have no rhyme or reason for which is labeled as the first and which is labeled as the duplicate. This matters because you then have to pick through the list to determine whether you're deleting a lower-quality or protected recording in favor of keeping a DRM free file.
All of these are relatively minor and the service is great after a day. It does take a while to match 15K files. If you have cable or DSL you quite likely will see quite a bit of time spent in the uploads as those services tend to be limited upstream capacity, but setting it up to run overnight can help a great deal.