I took a PhD course in the first year of my MDiv, and it was awesome; I have another scheduled for this year. Excellent way to network with other students, and if you do well, you'll be on the professor's good side for courses down the road. I would echo the comments above, though - try not to get in too far over your head. Most professors will understand that you won't have the full grasp that the PhD students do on the topic, but they will expect you to try, hard as you can. Also, if you haven't taken one before, PhD courses typically involve thousands of pages of reading. If you're on an otherwise tight schedule, a PhD course can wreck your other classes. Another note on requirements: often a course won't list language requirements; however, if the course is on someone like Barth, it would be very helpful to know German, etc.
Just my two cents. Enjoy whatever you do.