A must-have, no. I think rudder pedals are more important, because the first thing you need to do is get the physical movements sorted out so you can actually fly properly - so a decent stick with separate throttle, and pedals. But the TrackIR certainly does add a lot - I've just tried the Blenheim MkI in MAW, with that beautiful cockpit, and the stock SE5a in First Eagles, and it really does change things. It also makes it easier to fly into the ground, because you tend to lose track of the attitude of your aircraft... But I am impressed!