Just for I decided to see what RMSE would be if I just assigned each rating prediction a static value starting at 1 and increasing by .5 until I hit 5. Results below:

Rating RMSE
1 2.9016
1.5 2.4486
2 2.018
2.5 1.6274
3 1.3133
3.5 1.1407
4 1.1737
4.5 1.3979
5 1.7048

Popularity: 4% [?]