I have never seen this site before, but have seen others like it.
My guess is that this company does not provide tours (too many worldwide locations). What they are is a booking agent for tours. They will book you into organized tours conducted by tour providers in those cities and receive either a commission or add on a surcharge. You are dealing with the middleman.
Also note that they say tours are for 15 persons or more...so, what you are getting is a bus tour, just like what you get from the ship's shore excursion department--except, you may be going with people from other ships, various hotels and other sources...and they may take some time gathering up the people. I think back to tours we did with Viatour in Madrid on a precruise one year...they did pick us up from our hotel, went around town picking up others for a variety of different tours and brought us all to their downtown terminal where we all waited for our specific tour to leave--the tours weren't bad, just not as efficient as a ship's excursion where everyone goes to and from the ship.
Personally, my first choice in many cities is to contract with a local guide directly for a private or small group tour, arranged with other shipmates, and being for 10 or less.
If I were going to sign on for a large bus tour, I would not go outside of the ship's excursions just to try to save a few dollars. You at least know you are picked up and dropped off at the ship and the cruise line somewhat stands behind the product.
When you book through an agent, you have no assurance that the tour provider is actually going to be there or acknowledge you, no matter what the voucher from the agent says. And, if things go wrong, who do you have to complain to? Most probably work out fine, but what if they don't?
To me, it's not worth the "savings"...True private tours, from recommended local guides that you deal with yourself are a completely different story--individual attention and custom itinerary ARE worht something...