In my case... I have the money in the bank to pay off both car loans. I don't want to do that, because I want the savings there if I should suddenly become unemployed (or some other disaster). I'm paying the bank between 5 and 6 percent.
The 401(k) loan "costs" me less interest, and all of the principal
and interest goes into my own account. I take my own sweet time paying it off, knowing I'm keeping the money anyway. In the mean time, I've lowered my car payments by an aggregate $300 per month... which I plop down ads (more) extra principal payments on my house, saving even more in interest. In reality: I get clear title to both vehicles, my interest rate is zero, and I've just guaranteed a 5%+ rate of return (though no higher than that) on a small part of my retirement account.
All the while, I'm still contributing to my plan, still getting employer matching funds, nothing changed there. In the event of a disaster, yeah, there's risk -- theoretically I might have to empty the bank account within 90 days. If I'm unemployed for longer than that, things will get tight.
My ace in the hole is this: My company will not just dump me without a fair amount of warning, and "displaced" workers almost always get a nice parting gift - in my case it would probably a few months' salary. So let's assume tomorrow morning I get a phone call from my manager and find out I'm on my last few weeks of employment. The instant I hang up the phone, my next call will be to secure a home equity loan - to pay off the 401(k) loan, of course. Hey, I'm still employed. I'll
still be paying less than the original car loans, and I don't have to drain our savings.
If you do it right, and within a fairly limited set of parameters, there's very little risk and no down side. I still would not recommend a 401(k) loan under normal circumstances, though. It can indeed be a ticking time bomb if you don't know exactly what you're doing.
So what's this got to do with building an airplane? Everything! I don't borrow money for toys.
Meh. You're really only double-taxed on the interest you pay yourself... unless I'm just too slow tonight to figure that out. My tax rate isn't horrible yet anyway; I can live with it.