I dont know about the "really good" part, but if I am building a team I'll always switch to Flex. The next question/answer you are going to get -- good players can play any offense well. And then the biggest difference is Player Game Plan distribution. Flex can be spread around to be efficient, or can load 1 or 2 players a little heavier (15-20%).
But I do think there are Flex attributes you can recruit more to, and less of. You always want ATH, but I think in Flex you can take a bit less ATH for SPD. A big with SPD and able to Shoot is beneficial.