I was a regular here during my semester abroad (it's right next to a university) mostly because of…read morethe superb coffee and pastries. No kidding, during the four months I spent in Milan I never found a cappuccino or caffè americano that was as consistently good as what I got at PD.
As for the namesake paninis, the least I can say is you'll never get bored with the selection. The 50+ item panini menu ranges from plain-and-simple to exotic (even in the smaller, ready-made selections behind the counter), and while I didn't care for truffle oil, mashed eggplant, or walnuts in my sandwiches, there are some real gems in there (like #15). Bring a dictionary, though, because the sandwich descriptions are all in Italian.
My favorite thing to do was start the day with a cappuccino and croissant with grape jam filling before the mid-morning rush--during which time it's almost too crowded to move--and come back to study in the afternoon. Solid wi-fi, plenty of seating, and great staff (have I mentioned the coffee?) made the minor nuisances bearable. I can only hope that they open their next branch back home in Texas.