A beautiful palace, and an even better art museum. I went to the Borghese Gallery the day before, and as much as I loved that spot, the crowds were quite busy and the rules were strict. The Doria Pamphilj, however, was much more relaxed. It wasn't nearly as crowded (perhaps because I went in the morning), I didn't need to deposit my bag anywhere, and the staff seemed less overwhelmed and therefore a bit friendlier.
An audioguide would probably be a good investment! There aren't any signs next to the pieces giving any context or history. Just simply the name of the artist.
There's also an impressive collection here. Obviously, Velazquez's "Portrait of Innocent the X" is probably their most famous work. They have some early Caravaggio (my personal favourite), Raphael, Carracci, Titian, Brill, Lippi.
Although the collection is probably not as extensive or well-known as Borghese, I think the gallery provides a competitive alternative. I would also argue the palace itself is much more charming, and overall more beautiful and better taken care of than its rival.