Yes, the Uffizi absolutely cannot be missed even if you are only spending a day in Florence.
ABOUT BUYING TICKETS IN ADVANCE: I must add, that if you buy your tickets online, you'll be spending more money than you need to and the time slot you reserve may not necessarily be good--you may still have to wait in line. Those that phone in their reservations have a better chance of getting the time slot they want. Also, you run the risk of booking on an unauthorized website that claims they're selling tickets. There are some sites out there that are running a scam. The easiest way to reserve your timeslot and the most reliable way, is to call them in advance.
IT'S EASY: From the US, dial 011-39-055-294-883 during office hours (Mon-Fri 8:30-18:30, Sat 8:30-12:30, closed Sun. Italian time is six/nine hours ahead of the USA's East/West Coasts). Upon calling, you can select to hear the message in english, then you will have to go through an automated system answering basic questions and then you will be connected to a booking operator. They can speak english for you--just ask nicely.
This same phone number is used for all of the major museums in Florence, so tell the operator you wish to reserve tickets for the Uffizi, your preferred date and time and how many tickets you want. You will get a voucher number which you need to write down and which you don't want to lose! Write down the name that you gave them next to your voucher number. Bring the voucher number with you to the entrance and enter in door number 2. Do not get in the long line of people waiting to buy tickets. You'll present your voucher number and purchase your tickets when you enter.
I'd allow at least 3 hours in the museum. Get there early, have lunch and then head over to the Accademia Gallery--using the same reservation system I mentioned above. Makes for an awesome day spent.
This is the official website: http://www.polomuseale.firenze.it/ for more info. read more