I know 2 people that have 520s. A colleague and my gf.
My colleague had an overheating problem when making an OS update, from GDR1(Portico) to GDR2 (Amber). He took it to service and he got a new phone, one that also had GDR1. He then updated without problems. The problem never appeared again. His phone works perfect now. The phone now never overheats.
My gf just bough her 520 a week ago and I personally updated her OS and apps. Everything went perfectly fine. The OS is snappy, all apps/games run great, I can't see absolutely no problem with it. I have a 820 and even I am thinking of getting a 2nd hand 520 as a backup device.
From what I've seen, there are 2 apps that can't make up their mind. GetMeRated (a social photo rating app) and SophieLens HD (photo filter editing app). GetMeRated sometimes closes when she tries uploading a photo and SophieLens HD sometimes (1/6 times I would say) won't start.
Anyway, seeing how the majority of WP8 devices are 512 MB ones: Lumia 520/620/625/720, Huawei W1/W1, HTC 8S, I think both those apps might be just temporarily buggy, so no worries there. For example SophieLens HD just had 2 updates these holidays, one bugs fixes, one Christmas filters. Maybe they were in a hurry and a bug came up.
All in all, I'm saying that issues exists on every type of devices. Physical and software. Most of the time these issues don't exist at all. So go buy yourself a 520 and start loving it. Trust me, you will.