I visited the Ufizzi on a Sunday afternoon in June.
I used a Firenze card (purchased at the tourist office across from the SMN train station). No need for a reservation. No waiting in line. I walked straight to the cordoned off entrance (on left side/wing when facing the river), showed my card, and entered. You will have to go through security and have your bags checked. There is a double staircase going up to the galleries.
I noticed that about an hour before closing, there was no line to enter. And, the galleries had thinned out making it much more enjoyable to look at the paintings. Also, guards were less vigilant about those taking photos on their phones.
I would recommend entering as late as possible. Be choosy about what you want to see and really enjoy a handful of pieces rather than try to see everything. Focus on the most important pieces.
The Caravaggios have been moved downstairs and you don't see them until you are approaching the exit.
The cafe has a nice outdoor balcony with a good view of the Palazzo Vecchio bell tower. It is a great spot for a drink and some relaxation.
My tips: get a Firenze card to avoid reservations and lines, see a limited number of artworks, return for a second visit rather than do just one long visit, stop and treat yourself to a coffee and get some fresh air on their balcony.
One last thing...the gift shops seem to offer different items. I noticed that the jewelry offerings were not as good at the final shop before exiting.
All children under 18 - both EU and US - enter for free with an ID that shows the birthdate.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.