Basically, it's in the title — It works! Go play it!
Edit: If it's [i]still[/i] not working for you — that sucks. But it seems that the latest update has widely been a success according to various sites (i.e N4G, GAF, reddit, etc.) If you are having trouble finding a game after the latest up date, I suggest either trying Team Slayer as it seems to have the largest population, set-up your Xbox for DMZ host, or port forwarding.
http://forums.xbox.com/xbox_forums/xbox_support/f/9/t/214551.aspx
http://support.xbox.com/en-AU/xbox-one/networking/network-ports-used-xbox-live
I can speak for a few friends and myself in saying that search times are now that of the Halo 5 BETA (which is to say — instant). However, a problem now exists where every first game I load into crashes. I have suspicions that this is a result of being in Preview Program as I haven't heard this complaint as of yet. Either way, hopefully this will be resolved soon.
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Yeah, but people keep voting for Halo 4, so it's not totally working. I'm currently punishing players for choosing it one game at a time by destroying our vehicles, finishing off injured and hiding teammates (quickly of course so it counts as a kill for the other team and not a betrayal by me), and feeding the other team kills once those tasks have been completed. I'd suggest you join me. It's pretty basic conditioning; if enough people join me so that every instance of voting for Halo 4 can be punished quickly and effectively (and ONLY during Halo 4 so they learn to discriminate the harmful stimulus from acceptable ones), we will eventually be rid of the possibility of ever having to play that shitty game in matchmaking ever again.