I've got about 7 hours so far logged into this since Friday night.
If you've even been looking for a very polished STALKER experience....well, here it is. Performance has improved MAJORLY on the game engine - running it on 1024x768 w/ Full Dynamic settings. On my aging PC here w/ those settings, slowdowns don't even exist here w/ Call Of Pripyat - even w/ a fair deal of enemies on-screen, which is unlike their previous games.
The Faction Wars feature from Clear Sky - which seemed underdeveloped in that game - is completely gone, which is a good thing. The upgrading your weapons and armor feature found in Clear Sky is here. The artifact detector item from Clear Sky is back for finding artifacts. Hot Keys and Quick Slots have been made for Health Kits, Bandages, and Food. And yes, you actually now have to eat food to survive, here and there - which happens here and there, so it ain't an annoyance or anything.
You can talk to "guides", who often roam The Zone - and if you want to Fast Travel somewhere (of significance) in The Zone, they'll take you where you want to go for a decent sum of money.
There's now a sleeping system, so you can sleep in certain "friendly" areas to replenish your health and/or skip time so you can do missions that only can be done either during certain times of day. Also, you also have a place to stash your equipment in areas that are "friendly" to you - since you likely will come across a lot, if you kill and loot lots of other STALKERs for their equipment.
Even though you can save the game at any time - when doing a mission, once you are about to hit the "marker" on your map for doing that particular quest, the game does create a brand new autosave for you. That has saved me a few times, as I've got really into the game and...oh yeah, I didn't save...but, thank God for the autosave!
Some of the side quests - very memorable. To speak on some of the questing...
There's been a quest w/ a guns deal gone bad in this awesome shootout in a warehouse and a quest for finding the bloodsuckers' lair.
That one quest for finding bloodsuckers...Turns out a STALKER was playing everybody for a fool, set them up, couldn't face the fact you found him (when you do), and just offs himself with his own handgun right in front of you - suicide-style. Crazy.
If you liked/loved Shadow of Chernobyl despite its problems (technical issues and bugs), you'll dig that Call of Pripyat is a MUCH more polished STALKER experience.
EDIT:
I should note, the version of this game on Impulse required Tages Internet Activation.