Given that I've had all of 3 days to use iOS 6, I can only elaborate a little bit. I haven't figured out all the details myself yet, and some of the timing is a little hazy - it may actually predate iOS 6.
Specifically, I have one smart playlist that I purposely created to overcome some of the shortcomings of the smart playlist syncing under iOS 5. Since Match ignores whether songs are checked/unchecked, I essentially got tired of listening to random Christmas carols in February - so I used the genres, groupings, "kinds" etc to eliminate everything that I didn't want to listen to come up in a random shuffle (this included home recordings, garageband junk, odd audio recordings that weren't music, etc). It worked pretty well under iOS 5, though it never matched the exact number of tracks that iTunes showed. My aim was to eliminate certain tracks, and it did what I wanted.
Now under iOS 6 it's essentially empty. I think it may show songs that are local on my iPhone, if they meet the criteria, but since my iPhone is currently empty (it's new, just got it yesterday) I can't be sure of that. Some other smart playlists do show music, and I haven't determined what the criteria is that is causing this one to be empty.
Also, I did notice about a week or two ago - this is where it gets hazy - that the playlist was acting oddly, and showed far fewer tracks than it normally did (usually shows several hundred, it was down to a few dozen at one point). So I messed with my sync settings and accidentally downloaded a billion tracks to my old phone, so it's been all messed up for a little while - but the point is that the problem may have actually started before iOS 6 came out - it may be due to a change on the Apple servers, and I just couldn't see it because all my music was now "local" on the old iphone.
And I'm curious, if Apple were not to make any more changes to iTunes Match, and it would stay exactly as it is right now, with broken smart playlists, inability to delete individual songs, no cloud icons (etc etc from all the things I've read in threads), would you continue with it? Or is it the hope/expectation that Apple will fix these issues that keeps you using it?
Yes, I would stick with it, for both reasons. I do expect Apple will fix it ( for this to work correctly, iTunes, the servers, and the Music app all need to be playing by the same rules, I think we've got different sets of rules due to the staggered updates, and the October iTunes update will complete the process). And if it stayed exactly as it is, I would still stay with it because it essentially does what it claims to do - I can play my music from anywhere, without needing to sync all my files. OK, so now without my smart playlist I may have to cilck "next" when Jingle Bells starts playing, but worse things have happened to me in my lifetime. And it's only $2 a month, I can live with Jingle Bells.
"No cloud icons?" - actually there are fewer cloud icons. Yes, I think that's a step backwards but still a minor issue - instead of individually tapping cloud icons, I now have to tap "new playlist," then individually tap the songs, and then cilck the playlist cloud icon. Again, worse things have happened than needing the two extra clicks to achieve the same thing.
And deleting songs. Don't get me started on that. I'll just say it's completely unnecessary, and mostly desired because of a fundamental misunderstanding of how "streaming" works.