Hi Diego,
Although I didn't do the whole Rio Gallegos-Ushuaia trip, I did the Rio Grande-Ushuaia part by bus many years ago and I can tell you that if you are into mountains, woods, etc, this is definitely a must. This is the only part in Argentina where you can "cross" the Andes and be still in Argentina rather than in Chile, as Ushuaia is located on the other side of the Andes.
I agree that it might be ok to do the bus from Rio Gallegos, but I was checking and it's not 1000km but around 600km. Still, will take around 12hrs because of the immigration checks and the ferry cross. A lot, but doable. And there are regular buses from Rio Gallegos. As to the landscape, from Rio Gallegos to Rio Grande (in Tierra del Fuego) it's typical patagonic arid land. So maybe if you are interested more in the mountains and feel like exploring yourself, another option would be to fly in and out of Rio Grande, get a car there, and drive through the mountains to Ushuaia. The area of Ushuaia is very interesting and by having a car you'll save money in excursions. Whereas renting a car in Rio Gallegos and returning it in Ushuaia will cost you an arm and a leg. Alternatively, if you already have the ticket to Rio Gallegos, you could take the bus to Rio Grande and then pick up the car there.