I've been asking around as well. Everyone I've talked to has been pretty happy. The guy did mention that he got a pair that were heat cycled out and JPJP took good care of him.
Let's talk costs. A set of R6s cost ~$1360 ($1260 + shipping) new. So if you get 2 track days out of a used set which costs:
$400 - Tires
$100 - Shipping
$100 - mount and balance
Total - $600
So if a new R6 can last more than 4 days...new might be the way to go.
Feel free to adjust my numbers...
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red