Nice collection. I'm jealous. I missed out on the 15C re-release when it was available. I don't really have a use for it, but I really did want one.
I've got a couple of 12Cs and an 11C. I use them occasionally, whenever I need to do some maths that's not complicated enough to warrant firing up Excel but too much for my feeble mental maths.
I don't remember how I was lucky enough to find out about the 15C in advance, but I jumped all over that.
Several years ago I took a university final and the requirement was a non-programmable calculator. I looked on eBay and discovered how expensive an 11C would be, so I went to the professor and explained that I was by this point in life incapable of using an algebraic calculator and pleaded to be able to use one of my HPs instead. Thankfully he didn't care.
https://www.dropbox.com/s/tovgotxn0lmmjg2/File%20Dec%2001%2C...