Trust me, the first book is the lesser of the three, although I still enjoyed it. The second and third books are a totally different beast to the first, they blow it out of the water. The Dark Forest starts off relatively slowly but quickly takes off, and when it does, the action, the ingenious concepts, the pace of the story is exhilarating. Entire books could be written based on one of the concepts alone, yet the fact he used them all for one (three-book) story is amazing.
Admittedly, there will obviously be people who totally disagree with my opinion (though they surely can’t disagree with the ingenuity of the concepts), but if you enjoyed the body of the first book then I would HIGHLY recommend you read the next one.