You can smell the butter for half a block as you approach this place. Often, that's about when you hit the line for the front door as well. That said the cupcakes are utterly worth it. I'm not a big fan of their chocolate though, and prefer their yellow cupcakes with vanilla icing (often in egregious pastel colors). They're especially good a day later, once the cake and the icing gets a little firmer.
Magnolia has more than cupcakes though -- it has an array of traditional deserts including and excellent red velvet cake. Magnolia also does birthday cakes, but you must reserve your cake at least a month in advance. (Again, _at least_), or you will not be able to get one.
The line can be a hassle, and sometimes in the summer they even have security people managing it, but it moves fast. The best time to go is on a weekday afternoon -- no line and plenty of time to decide what you want.
Finally, that line may be a hassle, but it's great for celebrity spotting. _Everyone_ goes to Magnolia.