Literally just finished a 4-2 losing playoff series as a 6'4 PG (on the Magic, year 1) against Simmons. Not yet 90 OVR but, yeah, he's a hard assignment (was on Superstar difficulty).
I found some success by not going out to him around the perimeter - just sticking near the key at all times - and could usually cut him off driving to the hoop, but he makes some crazy contested layups, just going over the top.
He's also tough on D, but got my points mostly on set plays.
Also SteveQ, I agree on Fultz, he always seems to get difficult buckets when I played against him (in other MC's, he's teammate in this one obviously and therefore sucks).