I ALWAYS buy my vehicles and pay in cash. I save whatever I can manage to squirrel away into a savings account, and when it's big enough I know that I can buy a new vehicle if I want. Generally, I keep mine 5-6 years or more, so this works really well for me. If I need to do a big maintenance or repair, the money comes out of my new-car savings account, and it just adds a little time to my buying timeline. I figure, I'd rather pay myself each month instead of the bank.
BUT, my wife is a chronic "new car syndrome" buyer and usually wants to trade her car in every 3 years for the "next coolest mom-mobile on the planet." Honestly, I'm fine with that -- we're comfortable enough that I don't lose sleep over it, even though I wouldn't do it this way myself, and it makes her happy. Anything that makes her happy works for me, in general.
IN HER PARTICULAR SITUATION, we have found that leasing makes sense. GENERALLY, if you are always trading in at 3 years or so, you never pay the balance down to $0 on your note, so you always have a car payment, and you are absorbing the worst 3 years of depreciation -- the worst of both worlds. IF that's the case, then the economics of leasing can make sense so long as you don't get suckered into one of those "$399 lease (then in very fine print) with $5K initial down payment" deals. If the lease is one of the "first-month plus tax and title down" structures, it really isn't that bad so long as you are doing it for a particular purpose.