I dyno'd my 2001 TLS with and without it in back to back runs. There was no perceivable difference in HP. Some people think it makes the bike "feel" more responsive. I got mine for free (some mix up between pricing on the K&N PCII and the PowerCommander PCII... so yeah, this was some time ago) so I didn't mind trying it. I don't think it really makes much difference in total top speed, it may help roll-on, but it wasn't a measureable difference on my bike. At the time, they cost about $70/ea... if you check out the TL1000.com website, there's instructions on how to do it for the cost of 50 cent resistor.
I'm pretty sure it'll do different things to different models. On the old TL's ecu it "tricked" the bike into thinking it was in 5 or 6 gear all the time. I think that it may remove the soft rev-limiter on post '99 'busa's. It was supposed to do a similar thing to GSXR's. What's the claim on the newer gen bikes?