Tickets: The advanced ticket buying process is a pain in the rear, but I understand why they do it. When I was in the Galleria, I appreciated the fact that the museum limits how many people can enter and how long each person can stay. It was so much better than most of the other places we went, because we didn't have to deal with huge, noisy crowds.
Museum: The entire building seems to be decorated. It is as though each surface is covered in detailed ornamentation. I had seen many photos before visiting, but I could not fully fathom just how ornately decorated each wall, ceiling and floor truly is. You only visit two floors. There are a few key artworks on the second floor, but not that many. I suggest quickly passing through the second floor and saving most of your time for the Bernini sculptures, celing frescos, and Caravaggio paintings.
Art: Amazing! Amazing! Amazing! I just cannot stress this enough. The Bernini's are incredible and so much more magnificent in person. The Caravaggio paintings are more colorful in person. The ceiling frescos-well, let's just say that it is easy to understand why a couple of them are so important in the grand scheme of the art world. One of them directly quotes Michelangelo's color usage in a really obvious way. Just amazing.
Food/Drink/A Place to Sit: There is a moderately priced cafe. However, expect to pay extra and to wait a long time if you want a seat. You can stand and drink at the bar, like most places in Europe, but know that there really is no where to sit in the entire museum, except for the cafe and a bench or two in the hallway.
Gift Shop: Limited selection of items, many of which are lame or just not entirely relevant to the collection you just saw.
Tip: There is a Metro station closer to the Galleria Borghese, just outside the grounds, across the street. It is within maybe three or four blocks or 3/4 of a mile from the Galleria.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.