Not as good as its predecessor
Weapon and armor upgrades make all the difference now. You can choose different upgrade paths for most of your items to makes you more effective in either combat or exploration.
You have an option to join different factions to get better items and faction support and participate in faction wars. This is something that you would do playing game second time on max difficulty, as at first you want to uncover the story line.
Unfortunately, this is where Clear Sky falls short. Storyline is very straight forward and not as catching or involving as Original Stalker. I also got a feeling that Clear Sky is shorter on game play and has smaller area to explore, but that could be just my impression as I was expecting more.