They sure are nice, but we opted for none. As a couple, no kids, no pets, we've lived in ours up to two months at a time.
Whatever slides' reliability and leak status are, there is no doubt they add weight. I've become convinced the RV builders are less concerned about drivability and weight distribution than they are about the floor plan. Even an experienced builder like Winnebago builds some models with too short a WB to balance out a heavy walk-around rear bedroom. Take a coach you're considering to a truck scale and get front and rear axle weights. Compare with door jamb label ratings. See if an axle is overloaded or close to it. Make sure about 1/3 or more of total weight is on the front.