I've tried Starbucks in their cafes and at home. When properly extracted, their espresso is a good base for "big gulp" lattes since the heavy roast notes cuts through lots of milk. Personally I don't care for the edge on their blends, even when it's softened by milk and sugar. One of my neighbors is a big Starbucks fan, so I made her some lattes using their coffee. She commented it was the best she's ever tasted; the "secret" isn't in the blend, but the difference in the quality of microfoam. As a straight espresso, I found it harsh.
Even though Starbucks' coffee isn't to my liking, I do credit them with popularizing espresso-based drinks in the US. That's the good news. The bad news is they've become the defacto standard. The thread Starbucks Chairman Howard Schultz sees a threat to its brand elaborates on this point.