it sounds like the only way to get music on your iOS device is to download it from the cloud. however, once it is downloaded, it stays on your device until you specifically delete. then, if you want it back, you have to download it again. so, for the most part, it is a one-time download.
but yes, filling up a 32GB iPhone with music would take a long time over wifi initially...
I have used iTunes Match since it first was available in beta and it is not actually a "pick and download" thing. It just plays. So if you created, say, a "current songs" playlist and played it through they would be in all likelihood playable without an internet connection.
HOWEVER... if you scroll to the end of a playlist there is an option to "Download All." This should alleviate the fears about being away from a Net connection. Since we can now create playlists on-the-fly you could even create a quick playlist before, say, a flight and download it all before taking off. Downloads seem very fast, at least there seems to be no bottleneck on iCloud's side... yet (we all saw what happened when siri was opened up to the world).
A few other things...
You can see, when looking at your music in the Music app, whether songs are downloaded on the device, or only in the cloud. If you don't have Net access and are playing a playlist that is only partially downloaded it will skip the songs in the cloud. You can also, within Settings app, choose to only see music downloaded to the device.
Also, while iTunes Music synching is not available if iTunes Match is on, shared iTunes libraries are still available on the iOS device (local LAN only of course). But unlike iTunes sharing between PCs/Macs you cannot choose to download shared tracks to your own device.
Another thing... and I think this has been mentioned. But when you turn on iTunes match on an iOS device your current music library is NOT deleted at that time. Personally I don't think syncing to iTunes is all that important anymore so this is a good workaround to preload your cloud music onto iOS device: sync it up with music THEN turn on iTunes Match. Your synced music will be there for you until you sync to iTunes again.
Personally I am liking iTunes Match but YMMV.
BTW: For anyone still asking if iCloud music can come from iTunes when on the local LAN, whether WiFi or USB (really???), forget it! It's Internet only. But see above about preloading via iTunes before turning on iTunes Match.
Michael