I don't know why I say "surprisingly" nice, but I actually mostly enjoyed this museum. I took advantage of the free Sunday admission, as it seems a lot of others did.
First thing I noticed is that a lot of people were gathering in the main hall, I guess sort of an assembly or performance space? Wasn't sure what was going on, but around noon they had some live pianists playing music, which I thought was awesome. I didn't sit around to watch it, but the music added a nice atmosphere to the museum and echoed through the galleries, keeping chatter down.
They do lay out a bit of a chronological path for you, starting with the ground floor and making your way up to the [third] floor. The path is rudimentarily marked out with decals on the floor.
Sure, they lack a lot of 'famous' artwork, but what is art? Some of the work could be just as good as any of the crap that gets talked about time and time again from some of the other European or US museums. I thought some of the work was better, to be honest. But it's Serbian... no room for that in Art History textbooks right? On the other hand, there were some galleries that I blazed through, finding it uninteresting.
To each their own, but overall I was impressed and glad I went, as I had been on the fence about it.