Surprisingly addictive, if you can look past some limitations that didn't hamper other, similar transport sims.
It may not provide some enhancements that for example later Railroad Tycoon titles offered, e.g. all your railroads must be connected, there is no way to build diagonal tracks, any reports on your company or the competition are quite limited (even compared to the much older Transport Tycoon) and the competition is not as aggressive as it could be, often sitting on several millions yet not expanding its empire.
On the other hand, I found myself getting quite addicted to this game, probably thanks to the vast variety of vehicles (horses, buses & trucks, trains, aircraft, zeppelins, ships - even within each category there is sometimes almost too much to chose from) and cargo (ranging from classics such as passengers, mail, oil, animals, meat to more exotic stuff such as crocodile leather, olive oil and petroleum derivates).
Sort of a common criticism of mine towards most modern transportation sims, there are no random maps here but the scenarios are quite varied and there's a good choice of pre-made maps with a wide variety of industries and towns to connect in different ways so the lack is not as apparent as in some similar games.
I would say the graphics are sufficient for this kind of game, that's not what a transport sim is about anyway. In terms of music and sounds Transport Giant is quite comparable to other titles, nothing special either but here even more sounds don't really matter much for this kind of game.
In short, I would recommend checking this out but it's safest to start with some kind of demo and give the game some time to grow on you.
I'd love to see this company take some learnings from users' comments as well as adopting some additional ideas from Transport Tycoon / Locomotion or the latest Railroad Tycoons, that would combine the best of those worlds.