I've always had mixed feelings about those. The frame won't rust from the inside out, something that happened on some bikes from that era. They were a little lighter than they would have been with a regular oil tank (lighter = free horsepower!) and the weight of the oil was better distributed. I never had one, but I imagine they would run a little cooler, having more surface area to dissipate oil heat.
On the other hand, a bike without an oil tank just didn't look right and each frame joint presents another possible leak.
