If I'm planning on buying a Rome Pass, how do I reserve a time slot at the galllery? Also, is it worth it to buy a Rome Pass?
If you are going to use the Roma Pass at the Borghesi Gallery the only way to make a reservation is by calling them. I added up the costs of the places we wanted to go and it was cost effective. The ability to skip lines and ride city the transportation for three days was extra. It just depends on what you want to see and do.
Like Bob said, you call during their business hours to make a reservation. You produce the Roma Pass when you arrive. Because it is a timed entry, be sure to arrive 30 minutes early. You will have to check your purse, backpack, etc. too.
Be sure to arrive early (I think the Galleria Borghese asks for half an hour). When you check your bag the attendant will give you an opaque bag (like a gift bag) so you can take some small things inside with you (wallet, water, etc.).
I gave my hotel a time window and they booked the Borghesi for me. It's best to plan to be there as much as an hour beforehand. There will be a line to pick up tickets and a line to check bags, phones, cameras, etc. The line to enter starts forming at least 15 minutes early too. You may want to rent an audio guide or use the toilet too.
When you enter the galleries, go directly to the second floor. Most people start on the first floor, so you won't be crowded. Then you can go down to the first floor when most of the folks are upstairs. This can be confusing if you are using the audio guide, which takes you through the galleries on its own route (there's no signage for the guide in the galleries).
And don't let the extra steps in booking the tickets deter you. The Borghese is wonderful. And this is coming from someone who usually doesn't get too excited about art. I got back every time I go to Rome. It is well worth the (little bit of) extra hassle in reserving the tickets.
You have to reserve to get into the Borghese - and it is well worth the effort. Be aware that your time in the gallery is very limited - I think 1.5 hours. It's not that large but oh, what incredible sculptures and paintings! Note: No photography allowed. There is a nice walk through the park, along a quiet street overlooking the city, to the top of the Spanish Steps.
Rome Pass covers a lot including bus and metro. At 34 Euro, valid for three days, you could do the Borghese, the Colosseum and Forum and some of the Museums that come under the National Museums of Rome, plus the transportation; I think you would at least break even and it would save you a lot of queueing for tickets at the various sites.
Last year the time inside was two hours. You can walk through the park and follow the signs for Pincio, where there is a terrace overlooking the city. There's a footpath that takes you down to Piazza del Popolo and its "twin" churches.