Hey!
I've generally had no problems with TIM, but I can't really compare it to other companies as I've never used the others. While doing a major trip from Brasília to Foz do Iguaçu back via Curitiba and all of São Paulo state I had no problems at all and was able to use it very well. There are, however, some stretches inside the country where most foreign tourists will not be found, where there is little or no signal available (I'm talking about a region between Brasília and Belo Horizonte where sometimes one town to the next is about 100 km!)
It´s quite simple to use...but I didn´t get if you are wondering to buy a Sim Card or just choose TIM as operator. To buy you just need to go a store (there are plenty around the country).