An incredible museum, consisting of three floors and two buildings connected by an underground tunnel. No advance reservations are necessary; you can purchase tickets on site and there was no line, and no crowds because the museum is so spread out.
We stayed 3.5 hours and could have stayed longer, we just ran out of time. There are amazing sculptures, statues and art, including some very famous and well-known works. The view of the Roman Forum was unexpected to us when we happened upon it, and absolutely breathtaking. There are also great city views from the rooftop patio, and you don't need to be eating at the restaurant to access it (see below).
There is a cafeteria/restaurant with decently priced paninis. The cafeteria is self-serve - you order at the counter and take the food to an indoor table yourself. The restaurant is table service on an outdoor covered patio. We had lunch in the cafeteria section and it was a nice break from all the walking.
There are apparently audio guides available, which we realized part way through the visit, but we weren't offered the opportunity to purchase them when we purchased our ticket. I expect it would have enhanced our visit, as though there are some signs describing the works (in both English and Italian), more would have been helpful. As others have noted, the buildings are a bit of a maze, but in hindsight it was sort of nice to not have a linear tour, but rather to happen upon amazing things in hidden rooms and corners.
A visit to the Capitoline really should be in the list of top 5 things any first-time visitor to Rome should do.