I own three of them (two MK61's and one MK52 with EEPROM module).
I purchased them mainly due to my interest in Russian history and calculators. They are interesting artifacts. Mine have a "flourecent" LED display (green on the MK61 and Red on the MK52). They are very slow and have volatile memory (i.e. programs, vaiables and stack are lost when the machine is turned off). They are interesting in their rather unique way of displaying the program as you enter it. Across the single line display (from left to right) are shown: the "code" for the last step entered; the "code" for the previous step; the code for the step before the previous step; and finally the actual program step number of the last step entered (each separated by a blank digit).
The volatile memory makes them unfit for any serious programming work (in my opinion). Unless, of course you have the AC power cord, access to 220V 50 Hz electricity and don't mind leaving your calculator on all the time. By the way, the MK61 and MK52 both work off AA batteries. The MK 61 uses 3 and the MK 52 uses either 4 or 5 (I can't recall right now). So, powering them up without the AC cord is not a problem.
I've had fun playing around with mine, and have successfully "converted" several HP11C and HP32sii programs for use on them. To me, though, they are basically toys (but then aren't all the calculators I own over and above the one I use at work "toys"?)
As I mentioned in a previous post, I keep one of the Mk 61 units on my desk at work as a "loaner" calculator. Even those who know RPN are usually intimidated by the Cyrillic lettering on the keyboard. They seldom ask a second time to borrow a calculator.
I'd like to learn more about the module for the MK 52, which appaently allows you to store data so you do not lose it when the calculator is turned off. Unfortunately, the manual is in Russian, and my Russian reading comprehension is almost non-existent. Which reminds me; does anyone know how the module for a MK 52 works?
Take care.
Wayne