I haven't had the best experience with match, out of the 9000 or so songs in my library, I estimate at least 8000 are in the store, 7000 worst case. I had to upload around 4500 songs, which is kind of slow, and what's weird is it would split up albums randomly (eg. Tracks 1-6, 12 match 7-11 don't). Any music already on your iDevice is deleted, I can I've with that... My biggest issues are:
1. When you turn on Match, there is no way to upload songs via usb or Wifi, all of your music has to be downloaded from the cloud. (There are 5 iOS devices in my house, everyone has different 8-10k song libraries, we have comcast so a 250gb/mo cap.. we usually use around 200 of it between the 5 of us...I can see this becoming a problem)
2. When I add new albums to iTunes it takes a while before it sends them to the cloud (i'm talking hours). Even if I force it, the process takes around 15 minutes, and out of the last 3 albums I have added to iTunes, all 3 of them are greyed out on my iDevices. So coupled with the fact that I can't sync my music I find myself running into the situation of, oh this free EP came out, let me throw it on my iPhone and listen to it on my way work... Oh wait, I can't.
3. Sometimes my albums become corrupted on my device and I have to delete it off my mac and my iPhone, sync Match, re add it, sync again, and then I might be able to download it. Like I had a full album listened to it a few times, went back later and it was split into 2 albums (one called unknown album), and half the songs didn't work.
4. I had the brilliant Idea of making a playlist of the music I want to load on each device.. it was around 2000 songs... I was on wifi and tried to download all... My phone was unresponsive for about 2 hours and a lot of the songs were not downloaded (another reason I want syncing)
5. Over 3g it's very slow to download a song on the go, it will constantly cut out. And I see this being a problem with people who are not lucky enough to have unlimited.
I'll try 5.1 later tonight, i'll post my findings if anyone is interested.