I think you have to have two things before watching TOS...
1) An open mind
2) Closed eyes
That is, if you think the dated visuals will spoil things for you. The great script writers of TOS ;were primarily novelists and some (I would have to imagine) wrote for radio. Think of TOS therefore as one of the best audio books you will ever hear. If you're in it for 21st century visuals...just stop, don't bother and head to the theater.
But if you can do those two things and really LISTEN to the dialogue then...you're in for a wonderful treat.