This assumes iTunes 10.5.2, as this is the current version as of this writing. Doing a brain-dump of some of the more common questions that have come up, and the ones I've personally tested.
1) I'm stuck at StepX / it's taking a long time to complete.
This is probably the most common question I've seen here. There seems to be a few factors as to how long it takes the Matching process to complete:
1- Your library size. Self-explanatory - the bigger the library, the longer the process will take.
2- How many uploads you end up having. This is harder to gauge, but if you end up with a lot of uploads, it will take a lot longer for the process to complete. If you have a lot of albums that you know do not exist on iTunes, or a lot of old CD's that have since been remastered (see bullet 2) then it's a good guess that the process will take longer.
3- Your ISP's upload speed. If you have a lot of uploads, the speed to complete will be directly related to your ISP's upload speed. This is a second number they give you - so, for example, if your ISP's numbers are 15/2, it's 15Mbps down and 2Mbps up. That's 250KB/sec up, which is at best ~850MB per hour. So, assuming a large number of uploads, this could take quite a while to complete.
4- Overloading on the iTunes server infrastructure. This doesn't seem to be as much of a problem as of this writing. But when the service was initially rolled out in the US, and also in other countries, it was definitely a major factor. Whenever there's a new iTunes version or if a new country is added, I would expect potential issues for a few days. Also, I would check the iCloud Status page before running an initial Match -- http://www.apple.com/support/icloud/systemstatus/
2) Why is my very common artist CD rip not matching 100%?
There are many theories about this, but I believe the most likely reason for most common CD's not matching is mastering. You've probably seen the term "re-mastered" alot, but what it essentially means is that the sound engineers have gone back to the original masters and tweaked the sound in certain ways. Now since iTunes Match is primarily doing waveform analysis for matching, these slight variations between a CD master and whatever master iTunes is using could throw off the matching process enough to make it an upload, instead.
The Beatles' "She Came in Through the Bathroom Window" is a perfect example of this. The version the older 80's CD does not match, but the 2009 remaster does match. I'm convinced that it's because the iTunes database has the 2009 master and that there are enough differences between the two versions that the matching process is thrown off.
3) I don't want an album to Match, I actually want it to Upload. Is there are way to force an upload?
Not as of this writing (Jan 2012). I've requested a "force upload" option already and I would recommend that everyone do the same. An example of this is the Beatles in Mono - some of those tracks end up matching with stereo versions, which you may not want.
You could also do some very tedious waveform changes in a sound editing tool to alter them enough to force an upload, but it's really not feasible to do this on a mass scale.
4) My song Matched, but it's still showing as 128kps in iTunes. Where's my 256kps version?
It's in iCloud, but because iTunes does not delete your local version, you see that local version by default. Some people dislike this, but I think it's a good compromise as if gives the user the option to either keep all their local music intact or to delete them manually later. It's up to you. When you delete a song post-Match, you'll see an option to delete the iCloud version. If you choose not to delete that version, which is the default, then only your local copy is deleted and you are left with the iCloud version only.
Also, I would recommend enabling the "iCloud Status" column, in your Album List. It's very helpful to visualize what the status is of your songs. And here's a guide to what the icons mean: http://support.apple.com/kb/ts4124
5) Once the Match process completes, can I delete all my local music on my PC?
This question comes up a lot for some reason, so I added it here. The answer is that yes, you can delete your local files once they are in iCloud. But the question you should really be asking yourself is "should I?". The reason why I mention this are that there have been many reports of mis-matched files -- explicit tracks becoming clean tracks, mono tracks becoming stereo tracks, live tracks becoming studio tracks etc. In my opinion, it's still a good idea to have your original files backed up somewhere locally, like an external hard drive. But again it's completely up to you.
6) I have some vinyl and cassettes that I've ripped to MP3's - will these work with iTunes Match?
I have tested both and, imo, you should NOT expect a high success rate with these rips. There are a lot of factors in play and results definitely vary - in my case, I've found that things like pitch and speed have a direct impact on matching, so if your tape player or turntable belt is not locked in the right pitch your results will suffer. Also you need to match the start and end silence with whatever is used in iTunes, which can be a very tedious process with a lot of trial and error.
Overall, I'd say that if you are trying to match vinyl / cassettes as just a supplement to your main CD rips, it might be worth trying. But if vinyl / cassette matching is your main reason for getting the service, then you will likely be disappointed.
7) Is there a way to get around the 25K library limit?
As of this writing, the only workaround is to create separate libraries. This will at least allow the Matching process to run, if the library in question is smaller than 25K songs. But you still have an overall 25K limit (sans purchased tracks) in iCloud itself.
8) It says I can have up to 10 devices, but I'm getting a "5 Authorized Computers maximum" error.
You can have up to 10 devices, but only up to 5 computers. That sounds a little confusing, but think of it this way -- a system with iTunes is a "computer" as well as a "device", but an iOS system is only a "device". So you can't have more than 5 computers using iCloud, the rest would need to be iOS devices before you reach the overall max limit of 10.
9) My files are showing up as "ineligible".
Check your file format, your kps rate and file sizes, as there are some limitations. You can find more details here: http://support.apple.com/kb/ts4054
10) I'm not satisfied with this service - can I cancel?
The answer is "most likely no". In the Terms of Service, it mentions that the service is not refundable. But, If you feel that the service is not doing what's been advertised, I would suggest calling them and stating your case - there have been reports of some people getting a refund, if their concerns are deemed justified.
One caveat however, and something the rep may not tell you : If you do manage to secure a refund, you are not permitted to re-subscribe for one year. So if the services gets an upgrade later that actually corrects issues that caused you to initially cancel, you might have to wait it out to try it out again.
11) Where are the other Apple iTunes Match support docs?
check out this excellent post from JiminMissouri -- https://discussions.apple.com/thread/3619849
Hope this helps!