IMHO is overrated too. Comparing it to the other 3 games of the saga I found it a little bit disappointing. One thing that caught my attention were the worthless bosses. They have no charisma and they have to do absolutely nothing with the plot (at least with it's core). They are a side effect of how the world turned to be.
But I still think it's a great game. IMO, you can find there one of the most epic moments of the saga, and that is a battle of war machines when you aproach to the end of chapter 4 (I don't mention it clearly in order to avoid spoilers). Wow, that was one of the peaks. It's near to giving death to "you-know-who" at the end of MGS3 by yourself pressing coldly the trigger. Or near figuring out that you have been cheated by "you-know-who" near the end of MGS1 (via codec and all that comes after that).