This post will contain some minor spoilers. If you are just starting the game, the quick answer is yes, but there are some terms and conditions to this. If you are not at least 50% through the main story, it is probably wise to wait until later to read this post so there are no spoilers.
As I mentioned, it is possible to play the game after completing the last story quest in the game. There is a minor boss fight and some story revolving around HADIES. Once you have completed all of this the game will end. When it loads back up, you will be back to where you were before proceeding with this final mission, which kinda means the game does not allow you to play after completing the last story quest, but there is good. You get to keep all XP and any other goodies you earned during the games final quest.