Descartes had many valid points and of course some not so valid, as does every philosopher. First of all, Descartes and Socrates were both convinced that certain knowledge can only be acquired through reason. I agree with both of them since through reason, you can decide what is right, wrong, truth, or opinion. Again like Socrates, Descartes would not accept that he knew nothing. He claimed that we can only accept anything as being true if we can clearly and distinctly perceive it. This makes sense since this laptop I am using exists; I can see it, feel it, and obviously use it. Descartes started off by doubting everything, so that he can work his theories out from scratch, from a completely blank slate. Therefore, the fact that he doubted everything was the only thing he was certain of. This is where his theory gets complex, and lost me. However, I was brought back in by the expression, "I think therefore I am." This is something you and I have heard numerous times, I'm sure.
Descartes's next point I'd like to bring up is that there exists a perfect entity. He claimed that there would not be a perfect entity if it didn't exist and we would not possess the idea of a perfect entity if it didn't exist. God is very present in this theory, since God is that perfect entity, since no human being is perfect. By this, Descartes must believe that God exists, which I do not agree with however.
Philosophy is very complicated and each person has a right to their own opinions, some people are just more inclined to share them.