iTunes match allows you to get a digital copy of your entire music collection stored in the cloud. It costs $25 a year and you can access your entire music collection from iTunes or any IOS device and either download or instantly stream any song.
The initial process can take some time to set-up. A little longer than advertised. Itunes will provide an AAC digital copy for every song in your collection, but if you have music that isn't part of the vast itunes library, you will need to upload your own copy and that can take some time. However, it is much much quicker than competing services from google and amazon that require you to upload everything in your collection, which can take days or weeks depending on the size.
This is most useful if you have a large collection of CD's that you have ripped into itunes or if you have purchased mp3's from other services such as Amazon or Emusic. If you buy from itunes, you will automatically have access to any songs that you have purchased from the itunes store for no additional cost.
Songs seem to take a few seconds to start streaming so if its something you listen to a lot you may want to just download a copy to your ios device. This is also a good idea if you are on a limited data plan because streaming music will use up your monthly data allowance.
Overall, its a great service at a low cost and I would recommend it to anyone that has a large digital music collection. It provides a great backup option and allows you to easily access your entire collection at any time.