Truly excellent! I encountered a few buggy and it's not without it's flaws but certainly nothing I couldn't look past or navigate around.
I've just finished the single player campaign and I loved every minute of it. The sounds and music are excellent and the graphics are amazing, especially all the little details namely things happening off the sides of the playable map, etc.
Despite such a complex skill tree, mechanics and degree of customization, it is surprisingly easy to learn and very polished. I would heartily recommend this!