iTunes Match has worked flawlessly for me. I'd be interested to know what issues others are having.
For the first few days, it just didn't work.
When it did work, it failed to match many songs that it really should have, them being rips performed in iTunes from original CDs, and albums that are also on the iTunes Store(s). A few matched when I manually edited them for length - usually silence at the end of a CD track that has been chopped off for the iTunes version (tip for people to try…). Some are the same length but still refuse to match, even though they are clearly part of albums that all the other tracks match. tl;dr version: they need to improve the 'matching' part more. That's pretty fundamental to the success or failure of the service after all.
It is very hit and miss with syncing custom artwork (that is, any artwork added myself). It uploads it, but then doesn't seem to apply it to all versions in the music app on iPhone and iPad. You can have it showing in album mode, or in the thumbnail in the controls, but then not in the full album view…
The music app on the iPad is poop. Slow and what functionality is there is not very elegantly designed IMHO. The iPad version on my iPad 1 especially… I'd rather go back to the old music app and not have Match on that if they can't do better with it frankly.
Also, it's a kind of all-or-nothing deal with iOS devices - either you have Match on, or off, and if you turn it on it wipes whatever music you have added via iTunes in the 'old-fashioned way' - so you can only have the advantages and disadvantages of each method, when it's fairly obvious the user should be able to use both approaches if they want - I should be able to load up 1000 tracks before leaving the house via USB lead and then if I forget a couple later on use Match to just download those… not have to wipe everything and start again. It's just stupid.
Amazon lets you store 10 times the number of tracks for the same price. That's a problem, especially when using something other than iTunes solves the afore-mentioned issues from the terrible iOS music app implementation of Match.
Having said all that, I plan on giving it a second year when my subscription renews. But it has to improve by the end of that year, or I'm cancelling it.