It depends on if you have another reason to go there and if you are wanting to buy "regular" stuff like Mothers,
Meguiar's etc.
If you wanted to buy boutique stuff, you may have a hard time finding some store that sells it. Also, it wouldn't be worth the gas unless you were right on the line. You would be charged the equivalent taxes when you crossed back unless you spent the night (extra cost). And you need either a passport or some other enhanced ID card to cross now. Birth cert. and drivers licenses aren't good enough anymore.
But if you were wanting the "ordinary" stuff, and you were already going to be down there then yes, it would be cheaper to buy the stuff there.