I just finished a vanilla run of Clear Sky and, while my experience is still fresh, have started another playthrough but with ABR mod installed, since it promises to fix a lot of problems with vanilla, namely the faction war. Some early impressions:
-Bandits are now removed from the equation when it comes to faction war, they are the default enemy (no more Garbage robberies). In vanilla, because of them, faction war can never end. Now that's possible, will see as I progress. Good change.
-Just like in CoP ABR, combat is more lethal. You can die in 1 hit, but so can enemy. No more fights against bullet sponges in the early game. This still doesn't fix the absurd accuracy of enemies, so you'll still die a lot.
-No more stupid flash drive quests for technicians, all blueprints are unlocked from the start and all technicians can do upgrades. Upgrades have also been changed so that they make more sense. This doesn't trivialize the game because you're getting less money. Overall a good change.
-Stashes were removed, now they are only given as rewards for quests. Random stuff can still be found in crates, making exploration useful. It felt stupid finding a bunch of empty stashes around the world, so the change is good, now the rewards make more sense.
-Carry weight was decoupled from artifacts, now they only give stamina. Inventory is unlimited, basically, artifacts just make you run longer if you want that. This might be controversial, but having just finished a vanilla run, it's a godsend.
-Because of previous change you might think that you'll become Zone's janitor by collecting every weapon. Not really, because most income comes from artifact hunting, which is what makes sense in the lore.
-You can now sell mutant parts for money, and all mutants drop them. Killing mutants now makes sense. You won't get rich doing this early, but it's income nonetheless. I like it. Mutants are not just a silly obstacle anymore.
-All weapons models were changed, some new ones were added (F2000 with detector scope, yay). Most of them are from a combination of OWR, STCOP, AO, very nice mix and they fit well together.
-NPC models were not changed, thank god. A lot of modders are so autistic that they include HD models which stick out like a sore thumb. This mod changes things more tastefully.
-Less grenade spam from SRP, this is an amazing change after playing vanilla.
-Great optimization, this is something where CoP ABR excelled as well. Sacrifice is that some effects were removed and settings menu is hidden by default (can be enabled if you desire so, but the mod creator doesn't recommend it), but now you can actually get stable 120fps with few stutters.
Overall, I like everything compared to vanilla: combat is more lethal and rewarding, upgrades are now cheaper and make more sense, economy has been tweaked, faction war is not a PITA anymore. The mod has set out to make Clear Sky more of a shooter (which is what it was supposed to be, what with linear objectives and neverending faction war), and it mostly achieved its goals. I'd say this is the way to play CS for people who bounced off vanilla for whatever reason. Give it a chance. Yes, it removes some of the graphical charm, but the gameplay changes more than make up for it.