The butt-dyno reports an increase in top-end power. I swear that on my '99 I can hear the faintest of knocking when under load, winding it up to fuel cutoff (even with 93 octane). The NO eliminated this noise and makes the car feel like it pulls harder.
On the other hand, I know people who've replaced cats due to this, and I've seen what my plugs look like after using it. I'd recommend it only for the strip, and probably not for consecutive tanks.
I'm not a fan of fuel additives or cleaners, but I'll tell you, NOS works. I'm not sure checmically what it does, but it adds power, even for my '99 base w/intake (that's it).