Update: It is now called iCloud Music Library. Sigh. http://www.mcelhearn.com/where-has-itunes-match-gone/
I was frustrated with not being able to synchronise my playlists across all my devices, and have all my songs available on all my devices, so I decided to check out iTunes Match.
Verdict / TL;DR
Go for it.
What is iTunes Match?
A service from Apple that lets you synchronise your playlists and mp3s across all devices (laptop, iPad, iPhone). It checks what mp3s you have and compares them to the iTunes store. If it finds a track that ISN'T in the iTunes store, it uploads it. It then records what you have in your collection with Apple.
Obviously, this means that Apple will now know what tracks you have. Some people are concerned about privacy. My understanding is that Apple is not in league with the RIAA in the sense that it's not a trick to get you to confess what pirated tracks you have. Since Apple charge $20 or £20 per annum (I can't recall the pricing details), in a sense they're getting you to pay for your music.
What's cool about this is that even if you have crappy 128kbps files, they will get replaced (on your other devices, i.e. iPhone, iPad) with Apple's 256kbps files. In a way, it's a way to upgrade your mp3 collection to good quality versions.
Another cool thing about this is that Genius now knows what you have and it can make store recommendations for you as to what tracks you might like.
It works with iOS 5 and above, so don't worry about it not working with old devices. It will work.
Another bonus: removes ads from iTunes Radio.
1. Since it uploads files it doesn't recognise, and since bandwidth can be expensive, I suggest you delete duplicates. Click on Music, then go to View: Show duplicate items, and click on the duplicates and hit command-backspace to delete them. Obviously, you only delete one of the two duplicates. If it asks about removing them from the files or trashing the files, say yes. To be sure which duplicate item is the better quality item, rightclick the top bar where it lists the track bitrate and file size, and show the bitrate and file size, so you can tell which version of the file is better quality. Delete the lower bitrate, smaller files, as they're worse quality.
But pay attention to the song length as well. If the song length differs by more than 4-5 seconds, it's probably two different versions (e.g. live or a remix).
2. Go through your library and tidy it up. Make sure song names, track names and album names are all correct.
3. It doesn't like VBR files and doesn't seem to match them. Re-encode them as AAC.
1. It sometimes takes a long time to synchronise playlists or recognise them across multiple devices. You can try force it by adding and removing items to the playlist while online and while iTunes Match is running.
2. It takes a long time to upload and match, so be patient, and leave your machine running overnight as far as possible so it can do it. It took about a week to get mine sorted out.
3. To enable iTunes Match, just click on "Match" and let it get on with it.
4. iOS devices don't automatically know you've enabled Match. You have to manually go into settings: music and turn it on.
Things I'm not 100% sure about
1. It says that some of my mp3s are not eligible. I'm not sure what this means; apparently it's because the quality is lower than 96kbps. The best way to avoid this, as far as I can see, is to make 100% sure that your mp3s are not pirated versions; if you have audio CDs versions of some mp3s, delete them and record them off CD again (Insert the CD and choose "Import CD".). According to this page, 200 mb is too big as well. (http://computers.tutsplus.com/tutorials/getting-the-most-out-of-itunes-match--cms-21336)
2. I do not know where it stores the uploaded mp3s because I don't see a decrease in my iCloud Drive storage.
3. It doesn't seem to recognise some mp3s that it definitely does have in the store, and it insisted on uploading them even though several versions on several compilations are in the store. e.g. 1980s hits. I'm not sure how it assesses which tracks it has and which it doesn't.
Improving the quality of your mp3s
If you delete your original on your Mac/PC and then re-download it, it will give you the copy from the itunes store. It seems that what it does is it lets you download a better quality mp3 from Apple, but ONLY if the file in question is not one that iTunes uploaded, ie only if it is one that they do in fact have in the iTunes store. The way to find out is to Get Info on the mp3 in question. If it says that it is matched, it means iTunes has the track and you can delete it and download a new/better version from the iTunes store, I believe. Don't hold me responsible if this doesn't work.
if you rightclick the top bar and sort by iCloud status, you can then group select a batch of mp3s, delete them, and redownload them from iTunes (delete the low-quality mp3s that have the word "matched" next to them; these are the ones that Apple has a better copy of. If your mp3s are however 256kbps or better bitrate, don't bother, since Apple's ones are 256).