This is coming from a sound designer, and I'm not picking on free sound specifically at all, but "free" is not free, it just comes at a cost that isn't monetary.
Most of my colleagues and I agree on sounddogs.com as the go-to for one offs when we can't or don't want to start from scratch. They aggregate for a bunch of top shelf FX libs.
And as someone said, very often these sounds are jumping off points for something else we need to construct.
They're not free, but they're cheap. And you get what you need and you get out. So what's the value of your time and your work?