I guess I only gave the book 3 stars because John Green wrote it. It is by far my least favorite John Green book. 
 I liked Colin's character because he simply was about the idea of "mattering" which we all yearn for, but, in my opinion, wanting to matter is overrated and the desire to matter sometimes drives people to insanity. The idea to matter defines them to the extent that they do not see anything else but mattering. 
 I gotta say I could not help but feel a little offended by Hassan's character. First of all, I loved him, I thought he was hilarious, and I also like the idea of having a normal Muslim character in Mr Green's book. However, Muslims do not call other people "kafir" whether it be to a non-Muslim or anyone, and "kafir" by the way means being a nonbeliever in God. I also felt bad because Mr Green has portrayed Islamic prayers incorrectly as in the part where Hassan was praying early in the morning in a loud voice; we actually pray in a low voice, almost whispering. 
 Overall, I liked the math and I actually read the Appendix section, I really liked the footnotes, but I was a bit bored while reading it. Although I am not very impressed, time was not wasted on this one.