Siena

I would suggest at least 2 full days for Siena. Lots to see, and make sure you spend some time sitting in the
campo people watching!

1-2 full days is all that you need for Siena. You'll get more time in mornings and evenings since it is your base and few towns are as wonderful as Siena early in the morning or later in the evening when the tourists are mostly gone.

Everyone's different, but I preferred Siena to Florence. Siena is my favorite Tuscan town. No place like it, especially at night. Florence is...well...Florence. Lots & lots of tourists but one of the grandest cities in Europe.
Siena is closer to some of those other hilltowns, but Florence has more frequent bus & train connections. We stayed five days in Siena with no shortage of things to do.

We just returned from Italy on Sunday. Spent 4 nights in Siena. I've been to Italy a few times and Siena is one of my favorite towns. At the very least one full day in Siena is required, but more the better. Great food, wine and atmosphere and of course Il Campo. We stayed at Antica Residenza Cicogna and would highly recommend it. Good location and hosts were very helpful. We did the "Taste of Siena" walk one day through siena-activities.com (i think that's the site) and it was not overyly expensive and was very enjoyable. And Siena is also a good base for visiting Tuscan hilltowns via public transport.

Personally I was happy with one day in Siena, and I preferred Florence. Everyone is different! If you're thinking of doing one day focusing on Siena and one to take a trip to one or two hill towns, that would probably be a good idea.

We did 3 nights in Siena. Afternoon of our arrival saw the Duomo and soaked up the atmosphere. First full day devoted to a wine tour with evening to enjoy Siena passagiata and do some shopping. Second day we went to Monteriggioni and Volterra to get the hilltown experience, and then see the fortezza, Chiesa San Domenico. Did not visit art museums or try to do it all, and it was fine for us. It is a bit of a noisy city (sorry Siena fans!); worth seeing but not worth a week IMHO. Really enjoyed staying 3 nights in Montalcino, a perfect little hilltown! Recommendation of Antica Residenza Cicogna is a good one. Fabulous B&B!

We check into Antica Residenza Cicogna on May 30th! So glad to hear the great recommendation!

I'd like to hear what you think of the place when you get back - we're staying there for a week in September!

I loved Siena. One of my favorite places in Italy. We stayed 3 days in April. Stayed at Albergo Bernini. It is a good place to visit other hilll towns from....but there is no other place like Siena.