Robert Downey Jr playing a role he loves...there is nothing better. This is action hero perfection to say the least. Perfectly cast and perfectly performed. I will see it again...and maybe again. :)
Reading the book definitely would help understand a few things, but I don't think it would detract from a movie going experience if you didn't. It was fast paced and action packed. The very tame kissing scene may leave some Bella/Edward fans wanting more, but was adequately intense. It was also funnier than I expected. An ejoyable experience all around.