Like most people, I tend to use Google for 90% or more of my searches, but I sometimes wonder what results I'd get from other engines. Since I'm too lazy to open yet another Firefox tab and try it out, I make do with Google alone. With Zuula, I'm just a quick tab click away from seeing the results that another engine generates, and I don't have to re-enter my query. Very useful.
And as Debbie wisely points out, if it's good enough for Gina Trapani on the Lifehacker blog it's good enough for me!