I get comments all the time, indeed, using the 500 F4 seems to act as a people magnet which, by far in the main, I don't mind at all, though, has to be said, I've missed countless shots due to people coming to talk to me right when the bird I have been stalking for four hours plus actually decides he will play ball and do a fly past.
In the main, the most comments I get are about just how far I can see with the 500, as people think is has far, far more reach than it actually does.
On one side of a massive lake I get people pointing out tiny speaks of birds right over the other side, like absolutely tiny, whilst telling me that they reckon I could get a good eye shot of that one. Or, again, at a lake, they bet I can read the time on someone's wrist watch on the other side of the lake that's like 3km away.
I've had a few saying they bet I can see the moon with that remarks, which always make me smile.
Shooting airliners a while back from an airport observation area I got a family give me a seat number of their son and ask it I could get them a photo of him in he window as they think he would wave at them.
At the beach with a yacht moored way, way out to sea I got a couple ask if I was paparazzi and who was one the boat "is it Tom Cruise?".
In the end, if they are really proving interested, I tend to pass them the camera with the 500 bolted on so they can look through the viewfinder and they always look through and seem pretty damn disappointed with just how much reach it actually has (even when hooked up to a 1.4TC and on a 7D). They then change the subject away from its reach to how heavy the lens is.