I'll make a cake from a mix (Duncan Hines is my favorite brand, btw), but when it comes to frosting, I'd rather have my own, home-made. And then I do prefer buttercream!
This may be an appropriate place to say this, but I have recently tried the small "patisserie" cakes at Trader Joe's. One is chocolate/chocolate, which was a bit too chocolatey for me (never thought that would be a possibility for me, btw) and the other is vanilla/vanilla. And boy is that vanilla one delicious!! And it was the frosting that made the cake, although the cake itself was very moist. The frosting wasn't thickly applied, but it was very tasty, with a slight lemony tinge. I highly recommend this cake, and it only costs $6.99!!!