Job Interview Question

Though this was (most likely) not the most important question on my last job interview, it was the most difficult to answer for me. So why should I care? Well, because I'm wondering whether it's a common question and, if so, how should I prepare for it.
So, amongst heaps of other stuff (can't remember 90% of them!), the interviewers (2 of them, insisting) asked:

How do your friends see you?, What do they think of you?

I stared blankly.
Then tried putting some pieces together for some sort of answer.
I was still starring blankly.
The fact is, that question caught me entirely off-guard. Of all the questions in the world, it had to be one about how others see me(!). I was baffled. Not because it was an impossibly difficult question per se, but because I had no idea about what I could possibly answer. I wasn't prepared to use my creative skills that early in the interviewing process, and I had no pre-considered answer to give.
I tried asking for details: what sort of friends?, for how long should our friendship have lasted by now?, what should their backgrounds be, in relation to me?, etc, in a desperate try to stall. C'mon brains, c'mon, Think of some answer,...
... Any answer!!...

I did mutter some random adjectives in the end, as a last minute solution, and no, they hadn't been thought out either. [Perhaps this better explains my recent fascination with M.'s post on a certain topic.]

Aaaanyway, here's what I'm wondering: Is this an usual question? Should I get better prepared for its rising its ugly head next time?
If so, what sort of answers are expected from me? Seriously. I have no idea what any of my friends think of me, so I'm not prepared to answer it that way. By this definition, who's a friend anyway? Is it only about real friends, or does it include acquaintances too? Argh, I'm lost. Ideas, anyone?