Mine was really bad to the point it was dangerous- eg. change into second, go to accelerate and nothing happened for a second or two! Not good when you're trying to merge or pulling out in front of someone....
It was worse when cold, but there all the time. A hill I drive up everday I could feel it start to accelerate after I'd back off the throttle (!?); and then continue to subtley surge up the hill on a constant throttle. It wasn't noticeable before the extractors as the oxy sensor wasn't operating at all due to the old manifold somehow not being earthed to the engine (ie it was never trying to go into closed loop before!)
It's a lot better now and has more pick up in any gear at partial throttle openings and my fuel economy should improve too.
What Murray said is right about ~400 degrees- but that's the temp the oxy sensor needs to get to before it start to generate a usable voltage. When I was monitoring the output voltage on mine it only took a couple of km driving before I could see the voltage varying around 450mV indicating it was in closed loop and being used to control the mixtures.
I was also surprised how much time it was actually in closed loop mode. I was always under the impression that unless you were gently accelerating or decelerating then you were not using the oxy sensor output. In fact it's being used most of the time and only appears not to be when the throttle is fully open or fully closed.
I'll write a bit more on how to check the oxy sensor when I've got more time, plus some ways to test them.
It appears my detonation problems have gone now too; but will have to wait until the weather warms up to be sure.