I have had my D3100 for almost a year and a ½ and am very pleased with it. I cant comment about the higher ISO performance compared to the D3200.
In general I am satisfied with my photos at 800 or 1600. Very rarely have I ventured above 1600. I think a lot of this is subjective and more about managing expectations rather that being critical of performance.
The only two features I dont have and wish I did are a depth of field preview and bracketing. But in an entry level package I am not complaining and have no plans to upgrade for the foreseeable future. I am pleased with my photos and have a whole lot of learning to do before the D3100 is not enough camera for me.