I have the BlackRapid RS-Sport Extreme and the RS-4 with the BlackRapid Brad Underarm Stabilizer. Having used the RS-4 with and without the underarm support, I can highly recommend the underarm support (which replicates the RS-Sport Extreme's support design).

It's significantly more comfortable to wear your camera strap as a sling as opposed to hanging from your neck. The BR's design makes it easy to slide up from your waist. Without the underarm support strap, though, the padded shoulder strap can slide forward or backward when you pull the camera up and down a couple of times. The underarm support fixes that issue completely.

I'm definitely a fan.