I had an M-5060r, which from what I understand is the euro/multi-voltage version of the 506. It was alright. Aside from the dual mono configuration and the giant meters being fun to look at, I didn't find the sound was anything special. I know I'm in the minority and people love the big Onk's. I sold it for good coin and moved on to amps that made me grin instead of cringe.
That being said, I say go for it if the price is right. They hold their resale value.
Be warned, the big filter caps are a super-duper PITA to change out if they are problematic, and they are getting on in years now.