Andrew Robinson called it his "desert island speaker"... He used the Concept 300 standmount as his reference "bookshelf" speaker for two years, apparently, but says the 50 is better and probably better than the 500 even, for half the cost.
Ken Ishiwata had the Concept 500 (the more expensive predecessor of the 50) hooked up to the Marantz Ruby amp for demonstration room purposes...
Many reviewers are pretty enthusiastic. However, I haven't yet heard it in person, myself.