Like mentioned by a few, writing zeroes and ones or using some wiping program is critical when HDDs are going to change hands. Formatting doesn't really erase the data.
With that said, this was a good and informative topic to address and may keep someone out of a mess of trouble.
I've done that to a few. A nice chisel and hand sledge work VERY well.
If I'm going to toss them, that's the method I use.