I have it too, I use it to "upgrade" those low-medium quality mp3 that I can't get in lossless, I don't use streaming.
Pros:
*You can match your mp3s to a better quality DRM free iTunes Store AAC plus, and you can keep them.
*If you have your music well tagged, upload takes less than a regular upload because of matched songs.
*You can forget about sync music to your iDevice, since is in the cloud now
*You can "stream" music (it's a combo of stream and download from the cloud)
Cons:
*Your matched songs are upgraded in quality and format but it keeps your tags ( I would prefer let the user choice)
*If you select itunes match in your iDevice, you cannot play the songs you already have in your phone (don't know if are erased) So you have to choose between itunes match or traditional in-device music, which is not cool.
Overall I'm happy with the service since is not expensive and it upgrades those files I can't find in lossless formats.