I run 1.25" and it cleared my 305/70r16's. 1.5 is overkill I would say, pushing tire out far enough where a wide tire like mine could contact the fender when fully stuffed.
check out my build thread. link at the bottom. ive got 1.5 on just about the past 3 years and no problems. run great! even with larger backspacing wheels and fat tires. check mine out
I have 2" spacers on my rear axle because I have Blazeland long travel on the front which makes the front 4"-5" wider over stock. Before the spacers the rear of the truck would slosh back and forth in the tracks of the front tires. They still aren't the same width but closer and it tracks better now. I've had the spacers close to 4 years and never had an issues. But like JayRolla said, the tires can and do rub on the fenders at times. I try to limit up travel with some ZUK coil springs and Energy Suspension extended bump stops