If I'm jonesing for a good maggie, I always sit at the bar and participate in the construction. Otherwise, you're going to get lots of sweet and sour, and even worse, Rose's lime juice.
Like Sean, I make my own, using 4 oz. decent tequila (Hornitos or better), 2 oz. of my own blend of lemon juice/simple syrup, 1/2 oz. cointreau (or to taste), 1/2 oz. OJ, 1 oz. fresh lime juice (or to taste). The key for me is that the maggie be 1/2 tequila, so you can actually taste it. Stole and then tweaked this recipe from the champion mixologist at the Bellagio (more tequila, less cointreau and OJ), and we love it, though a second may require surrendering car keys.