The time it takes to find a game (less time = more likely to use), the map pool (too many naval maps like there were a month or two ago and I'm less likely to use it), and how long I think it'd take to find a game via hosting a custom game.
The number of players seeking a match will also make me more likely to use it (since it indicates it's likely to be less time to find a game)
The change giving people timeouts for disconnect failures also made me more likely to use it, since there's been a dramatic decrease in the number of failed games once a match has been found following that change compared to before.