The main benefit to irs is the fact that drive shaft doesn't ever move. A properly built independent rear suspension vehicle will also handle better than one with a solid axle. Narrowing irs is actually cheaper and simpler than solid axle trucks. Often times, cutting the factory shaft and replacing one yoke is all that's needed to narrow it.
The biggest cons to irs is the doubled amount of ball joint maintenance. Tire wear is easily eliminated just like it is in the front suspensions.
The square body Dakota was talking about is a local guys truck. The spindles, arms, drive shafts and rear frame section are all custom made, the actual pumpkin from a jag was the only factory irs parts used. Where issues arise are when people try to use the entire irs from a car on a truck. You could easily build an irs setup for a full size truck and just use 4wd/AWD hubs and spindles in the rear.