Prompted by the thread on overtaking speeds, and interest in what the advanced drivers here would do. This is less about whether overtaking is safe, more about the thinking behind a decision to take an overtaking opportunity or not.
Okay, the answer does depend on the road, conditions, type of car you have, type of car numpty has... type of numpty as well for that matter.

So I'm going to describe a situation I find myself in from time to time:
- Fairly flat and straight NSL single carriageway road
- Road surface in good nick
- Dry weather and clear visibility
- Moderate traffic in both directions
- Obvious performance advantage over numptymobile (not necessarily NSX vs Trabant, but you know you can accelerate quickly enough to pass and return to the left safely)
- You know the road well, and for the sake of argument there are no side turnings on this stretch and no dual carriageway coming up
- Variable speed numpty who sometimes does about 60 but is usually closer to 45-50, and also hits the brakes every time he sees a car coming towards him
- Numpty has a gap the size of a couple of runways in front of him
- You are being delayed but it is not critical to get past (imagine you're on the way to a meeting you don't want to go to with people that you don't like, but the boss will yell if you're late)
Now, on the one hand there's a case for passing assuming you have a safe opportunity to do so, and since there's no one else ahead of him you can put some distance between you. On the other hand I sometimes feel safer keeping him where I can see him, which means staying a safe distance behind him. Plus you don't have to worry about him suddenly deciding to accelerate again just as you draw alongside.
If the guy looks like an accident waiting to happen it almost comes down to whether it's better to blissfully unaware since you passed him ages back, or to have it happen far enough ahead of you that you can stop safely. When I've got all the time in the world I tend not to bother overtaking, but in other circumstances I normally prefer to get past and put half a mile or so between us. But it's a bit of a toss-up sometimes, and if my usual inclination is a poor choice I'd rather hear it from the IAM mob here than find out the hard way.