How does the 3G provided by ISPs work? I checked Terranet's offers, from what I understand you can get the data SIM from a store that has it, and then it's a little unclear how you actually chose the plan, how you can have an account with terra that you can later check...

Does anyone know how it works, how good/bad it is? How is the coverage compared to the mobile companies? I suppose the ISPs get their 3G through the mobile companies so how is that reflected in the 3G quality?

Thanks
They use touch and/or alfa's network. So the advantage you will get is that if the ISP uses both networks you will get the coverage map of both. You should call the ISP and ask which network(s) they use.