I had friends in last month, though, and they were interested in visiting this, so I went along. The archtecture of the museum is interesting enough, but the name, to those used to the National Gallery in either London or Washington, DC, is misleading: rather than being a wonderful collection of portraits, the Alte Nationalgalerie is an eclectic collection with the majority formed from the personal collection of J.H. Wagener, who donated 262 paintings with the proviso that they be retained as a single collection and housed in the Nationalgalerie. The collection is primarily Romantic and Classicist and for me, a little goes a long way. More interesting is the extensive collection of 19th century statuary, as I have a fondness for Canova.
This museum does not touch the Neue Nationalgalerie (an amazing museum), and I think that what my friends were looking for in terms of collection would have been better served by the Gemäldegalerie, which I will visit in their honor next month.
The most interesting take-away from this museum is how the Romantic period looks so incredibly modern: very many of the paintings could be used as covers for contemporary fantasy, paranormal, or "literature" novels.