No offense to the guys that like belly spinners, but I think the in-line orientation of the swivel on the RoadRunner/pony/horse head makes a difference. I've seen underwater videos of them in action, and the way the head is weighted and the drag from the blade makes the eye into a pivot point. They run with the nose down at an angle and the swivel on a horizontal plane. This means there's less friction on the swivel as the bottom loop of wire doesn't rub against the inside wall. The hook rides up, keeping the body and tail material from interfering with the blade.
There are a few things about the official RoadRunner I don't like. The hooks, for one. The regular RRs come with bronze hooks, and I don't like bronze because it rusts easier than plated hooks. I've seen RRs in the package with red hooks, but it looked like the red coating was flaking off. I also prefer sickles, and they're not an option. TTI-Blakemore has also gotten sloppier about the paint jobs and keeping paint off the swivel. I've gotten a few that had paint clogging the swivel and keeping the blade from turning, and I don't like having to clean them before I fish with them. I have a bunch of RRs because the local Cabela's had packs of 3 heads on clearance because the paint covered the hook point and no one would buy them otherwise. The paint ended up being pretty easy to scrape off, but I wouldn't have bought them if they hadn't been cheap.
Belly spinners are great when you don't like the official RoadRunner and don't have one of the imitation molds. But IMHO the RR head style is superior. I prefer a quality horse head with sickles to the real thing, but with the company cracking down on individual sales I'll probably end up starting to pour them myself eventually.