I was really surprised after all the great early buzz that the reviews came crashing down.
Honestly, they're a little harsh. It's no masterpiece, and quite uneven but there are great things about it, the acting particualrly from the returning duo of Rush and Blanchett is top notch (big surprise) the costume design, art direction and cinematography are quite good as well, and while historically all over the place that battle scenes were well done. The score was an issue, as it fluctuated between all different styles and levels of being excellent and complimentary (the violins during the battle), and obnoxious. The film's major problem is that it lacks the exciting and well paced political plot of the first, and instead of balancing personal lives and political struggle like the first did, this panders a bit much with soapy relationships, whereas the other aspects of the film are done well. Overall its worth watching, if a little less well constructed than the original.
Also, Samantha Morton (who I'd forgotten was in this) gives a great performance as Mary Stuart in a small role.