Does it give a useful field of view?

Scanning up/down, left/right, gives you a large field of view.

Is it hard to get used to?

Just like using a rear view mirror, there is a learning curve. You first learn to trust it, then you learn to not depend on it.

Does it block your field of view?

It is a lot smaller than an aerobar mounted water bottle.

And yes, that’s where I mount my water bottle!

Would you move your water bottle for another 0.5 – 1.0 MPH?