Singleplayer is good as always but what really makes this game fun is sipmlified multiplayer.... It's great.
But what really makes this game worthwhile is multiplayer. Unlike Chaos Theory or Pandora Tomorrow here you have much less gadgets, less shadows to hide in and overall it's less complicated to play. It's easier and funnier than any Splinter Cell before. Maps are big. As a Spy you will usually be high above your enemy or in ventilation duct. As UPSILON Force you will be forced to take long road around everything but unlike spy you will be armed with weapon and unlimited amount of bullets.
So while singleplayer is average, multiplayer is what makes this game great.