I have 2 relevant AppleIDs authorized for the music on my iTunes (ONE old long-time Apple ID for iTunes purchases, bound to pre-mac.com email; ANOTHER newer mac/me.com ID which had mostly been used only for App store stuff until iCloud -- but which I figure has to be the one under which I try to consolidate in the future).
I've taken the plunge (sprang for Match when I was signed in with the newer account) and find that most music from BOTH accounts is getting matched. Turn on the "iCloud status" column in iTunes -- and sort with that column header -- to see what's "Matched" and what's "Uploaded" and what's "Ineligible" (or "Error"). [You can also make "smart playlists" based on iCloud status, which helps troubleshoot.]
I found two albums worth of "ineligible" Music files -- turns out I had forgotten about a third ID at one point about 8 years ago, those two albums were attached to that one. HYPOTHESIS: it won't upload stuff for an account that's not "authorized" for the iTunes incarnation that's hosting the process. I no longer have access to the email, so I assume I can't authorize those.
WORKAROUND: if you have lots of music from old accounts, look for TuneClone (3rd-party app that "burns" virtual CDs from m4a and m4p files, so you can re-import them as if they'd come from CDs.
I'm still nervous, by the way, about Match's warning that it will "replace" the existing library on my iPhone (despite assurances online that nothing is actually removed), and I've enabled Match only on my (small-capacity) iPad for now. Playback requires a few seconds' patience, but I'm pleased to have access to all my files.
Now with a big hard drive on the new iMac I'm finally motivated to import many of those old CDs of music that I knew wouldn't all fit on my iDevices (nor, for that matter, on the hard drive of my older machines).