How good are the American carriers really??

I've been scolded here a coupe of times for talking bad about American cellphone carriers without having that much info about them. The only info I got is actually from podcasts from engadget and here, and they like to moan a lot about the American cellphone carriers :)

So these are some questions I would like to have answered from you:

*How fast are the real speeds when you surf on your cellphone? Both with LTE and 3g?

*How much does a contract with a data plan cost and how much can you surf then?

*Is it really true that you have to pay when somone calls you?

*Why does so many cellphone companies have to customize their cellphones for the US market?

I think it would be much easier for me to be objective about these questions if people who actually lives in america could answer these questions for me.

Thank you in advance :)