50,000 words in a month is I suppose a reasonable goal if you can set aside the time, and if what matters to you is demonstrating that you can write 50,000 words - it would be very much on the short side for a published genre SF/Fantasy novel. And knowing that you are able to write at length and finish things is definitely worth having.
I don't do NaNo myself, I just plug on with the same few thousand words every Friday night that I do all year. Which seems to work for me in getting things finished.
November's a depressing month to be thinking of doing it in, though.