Wouldn't exactly call it capable, it had 20 memory registers and 106 bytes of programming memory. Plus Reverse Polish Notation is not exactly easy to use. Still great memories, so many kids of my generation discovered computers through those.
I actually got one from eBay last year, cost me about $60 with 220->110 converter. I think somebody's selling unsold inventory from the factory. Printed out some old articles and went through first 2 legs of Kon-Tiki journey. Fun times :)
Yes, MK-52 and MK-61 always had a large community of tinkerers around them, exploiting hardware and software bugs (or rather the machine's extreme simplicity) to write seemingly impossible programs. The bugs were never fixed in later revisions though, so while they were fun to play with as a kid, I have mixed feelings about it as an adult engineer. Having an errata instead of actual bugfix wasn't very helpful. My uncle hated them back in the days for this, and when home computer kits appeared, he got one and never looked back.