I have the HP-50g. Love that thing, though I don't get to use it much these days. I bought it for taking the tests in my intro physics course at uni, after my TI-89 got stolen by some doucherocket. My physics professor was a huge fan of the HP RPN calculators, and recommended them highly.

I was also notorious for bringing a slide rule into tests in case the batteries ran out on the calculator.

Yes, a number of people collect them. But a quick scan of Ebay would show that.