I have used clocks very infrequently ... and it drastically changes the nature of play. RP goes way down for one, as players see it superfluous to "solving" the mission. And avoiding combat goes way up because it's such a ridiculous time sink compared to any other mechanical part of play. Characters will also, for example, pay bribes of an amount they never would otherwise because the meta-incentive of the out-of-game clock makes haggling a bad thing, even though that goes contrary to how they would normally play their characters.
The fact that players put back in RP as soon as there is no longer a clock leads me to the (somewhat obvious) conclusion that the players I play with enjoy doing it. So overuse of clocks would be counter-fun for my table in that aspect. But still, occasionally moving people out of their comfort zones is also fun.
I've also done short in-game clocks, but that has much different results. For example, when 30 minutes of combat is 24 seconds of in-game time, that's not a problem. Taking rests is a problem. Which screws up inter-class balance for short-rest-recovery classes like the warlock.
I'm much more likely to have longer in-game clocks (days+), such as a count down to a celestial conjunction or something.
