I have a pair of 50's and until recently a 60. Live a few miles away from the original plant and have come to know quite a number of "old goats" that use to work there.
To answer your question, after shooting many thousands of rounds out of quite an assortment of Reisings we look at the following items of concern from a functional standpoint:
1. The firing pins have a habit of breaking when subjected to consistent hard use. I believe Ken Kristie has an "unbreakable" model which we used in a 50, ran it very hard and it did not break.
2. One of the charging handles developed a crack in the left wish bone portion.
3. An ejector popped out but can be reinstalled quite easily. A new one should be used.
4. Parts are to some degree hard to come by. Don't buy used parts.
5. The Wolf spring kits work very well.
6. Some of the feeding jams with factory 20 round mags. occured because of weak magazine springs. We took some springs out of 30 round Thompson SMG magazines, cut them down a bit, contured them to the Reising follower/system etc...and this cleaned up that problem. The 30 round Ken Kristie mags worked flawlessly. While on magazine issues, disassemble your mags, scrub out the inside of the body/spring & follower and then coat them with a teflon dry lube. We use Remington Wonder Lube up here and it works great. Finally, examine your feed lips. Make sure they are properly contured to the feeding of the cartridge into the chamber and that there are no nicks, dents etc. in them. Check the mag. bodies for dents etc. too.
7. Like a M1 Carbine, do not relieve the feed ramp area. You may unsupport the rear of the case body resulting in an explosion.
8. We are not very confident in the extractor but they seem to hold up fine. I check mine quite frequently-examine the cleanliness of the system, contour and spring tension.
9. Reisings are not dirt friendly. This is not an AK. We fired AK74's with what would be normally considered an extraordinary volume of beach sand in the receiver and it would still function. We doubt this would occurr with the Reising however we are not about to perform this test with a 60 plus year old C&R selective fire sub gun valued in the mid 4 digits in USD's. The USMC did this at Guaducanal under extremely testing conditions and the Reising system was a failure. So you should keep it very clean and lubricated.
Other than the above caveat's to be aware of, if you are considering the purchase of one, I'd have it disassembled by a competent individual and examine the firing mechanism for wear & tear, springs for tension, condition of the bore & chamber, charger, fit of the compensator (if any) & magazine funnel and the condition of the wood stock. You should fire one first (maybe your future one??), also to get a feel of the system. It's got a lot of "pop in it's soda bottle" for a sub gun of this configuration.
The final conclusion is that this is an extremely fun sub gun that works quite well with the above items being looked at. C&R status adds a lot of flavor to the concept of inter state portability. Never mind what they sold for a year, two or three years ago. They are a low cost/end entry level sub gun that can provide megatons of fun for the beginner, expert or anyone in between.