[Ode to a Grasshopper]

Seriously, I voted pro-spank, but as noted there needs to be more variables included for a representative answer. On the one hand I know there were times when I deserved a good slap on the backside as a kid, and I probably should have got a couple more than I did (though by the same token there were one or two occasions I got one where there would have been more appropriate forms of punishment). I'd sooner try explanation and example over spanking, but that doesn't always work, especially with younger kids who can't necessarily understand the consequences of what they do. It'd be nice to think I'll never hit my own child/ren should I have him/her/them, but that seems a little unrealistic...
The main drawback with spanking my childhood self is that most of the time the lesson I came away with was "don't get caught" as opposed to actually considering why what I had/hadn't done was 'wrong' (usually because it was something that could have caused me serious physical harm), so it was kind of counterproductive in a way.

Short answer: The punishment should fit the crime. Spanking one's kids - only where warranted, and only in moderation - is okay by me.