There are two passes available for Siena, the Opa Si pass and the "Siena Pass". Both of them give 50% discounts, and neither of them sound like they save any money for the traveler. We will visit Siena in May - staying in an Agriturismo. At that time of year, are there advantages to having either pass? (IE are the crowds so dense you can't find the ticket windows, for example?)
Frank
PS I should add that this year Opa Si pass is 27 Euros, admits you only once to each attraction of the Duomo complex, and requires, according to another blog, a 5 Euro reservation fee. It is more expensive in the time in late summer when the floor of the cathedral is uncovered. The Siena pass (http://www.aboutsiena.com/siena-pass.html) "starts at" 17.2 Euros per person and allows for half off on admission prices "without standing in line." (How do they do that? Do they have 2 lines, one for pass holders and one for non-pass holders?)
Yes, there's a designated line for passholders at most places of any size; if there's only one line, just walk to the front of it with pass in hand and you'll be waved or scanned in. I'm not sure what your interests are, but I looked at the passes and decided it wasn't worth it to me. In May, the only line you will likely encounter is at the Duomo and it moves quickly.
Here's Rick's take on the Combo-Tickets: Siena always seems to be experimenting with different combo-tickets, but in general, only two are worth considering: the €12 Opa Si combo-ticket that includes the Duomo, Duomo Museum, Crypt, and Baptistery (a savings of at least €9 if you plan on seeing all of those sights; sold only at the ticket office just right of the Duomo, near the Duomo Museum entrance), and the €13 combo-ticket covering the Civic Museum and City Tower (a €3 savings; must purchase at City Tower on Il Campo).