The Big C has it's moments when you can just let go and laugh but the majority of the time it revels in its own sadness. It would be difficult for a show that is based on this subject matter to not be somewhat depressing but overall there are moments of light. Although it can be quite dark that does not mean that it's not an entertaining show. It's very insightful and seems quite realistic. I know if I found out I had cancer I might behave this way.
I personally think this show only needed 2-3 seasons max as it got a little slow towards the end but it was still pretty good. But honestly give it a chance as it is quite inspiring. Other peoples sadness and misfortune has a way of making you reflect on your own life and realise what you truly have.