It's also the number 1 selling HD EFI tuner for years too 
But yet Harley MoCo, who paid for it's development, chose to drop TTS (or whatever it was called) and go with another company.
I have an older TTS, a newer TTS, a PowerVision and a Thundermax. I have no issues using any of the devices or in the case of T-Max their ECM and software.
As has already been said, the TTS can be difficult for many, and without a dyno the upper regions are simply guesses. And it requires a DIYer to ride around with a laptop rigged up to their bike.
Powervision is really easy to use, somewhat expensive, but has terrific support for the few that need it. As has been said, riding around with a small graphic interface (or laptop with TTS) and trying to pay attention to cell data while riding on a public street is dangerous and should be/probably is illegal.
Thundermax is super easy to use, tunes the entire range, uses wide band 02 sensors, is super easy to use. Another added benefit of Thundermax vice TTS is T-Max will not lock the factory ECM. In fact, Tmax doen not even use the factory ECM.
As for cost, add the price of TTS to the cost of a full dyno tune, and the price comes to around a $1,000, Powervision will not require a dyno operator so it's final price comes in slightly lower than the total cost of TTs. Thundermax with Autotune also does not require a dyno operator and can be had for mid $700s.
Unless a DIYer just likes to tinker, or has to have every last little bit of whatever out of their bike and likes to pay dyno operators large sums of money, Thundermax is the way to go. Costs much less, easier to use and for 99% of the owners and their bikes it will produce the best and easiest results in the least amount of time.
Final thoughts. The OP would be best served to shy away from TTS and look at either PowerVision or Thundermax.
About me: Just a DIYer with way too much time on my hands and a love of motorcycles. Unlike several that have posted in this thread, I have no commercial interests in any of these three products and am not here to sell anything or advocate for anyone or anything. Please keep this in mind as you read the various comments in this thread.