I just picked up yokozuna cables. I really like them, but I also got them at a team discount instead of msrp. They're pretty good, and I would say you can feel a difference, however it's not necessarily a performance difference, just a slightly different feel. They seem to eliminate a lot of the friction between contact point (lever/shifter) and it's end mechanism (brake/derailleur). Brakes have a little more feedback to the levers and ever so slightly decreased effort when applying the brakes. Shifting is a little better, too - rear derailleur is a little stiffer (but due to the mechanism/amount of throw required, it's not that pronounced). However, front derailleur feel is way better. Shifting is noticeably stiffer and more responsive, and not "mushy" at all. Shifting under load is noticeably improved.

Full disclosure, I also had replaced the stock cabling as well, which also eliminated some extra cable length around the cockpit (I dropped the stem several spacers from stock configuation), so that also may have eliminated some play from the stock setup. That being said, cable housing is VERY stiff, and I'm not doubting the "compressionless housing" claim they make.

All in all, they're nice, maybe even a slight luxury. I'd say, if you're in the market for new cables, definitely give them a try. They're not that much more expensive, and for something you only upgrade/replace once a season, it's not entirely cost inefficient to give them a try. If you have working cables currently, use em till you need new ones, and then give nice ones a try.

EDIT: oh, I also have Rival, if that makes a difference.

My cables are too new (2 weeks old now, I think), so I can't comment on long stretch/break in period, but if you remind me, I can let you know as time goes on. However, in the short time I've been riding them, I can say that I think they've needed less adjustment than stock ones. After my first weekend of racing (immediately after I got the bike), I needed to tweak the barrel adjusters on the stock cables (particularly FD). Now, after two weeks of training with the Yokozunas, I still haven't had to touch the barrel adjusters.