First off, do not go into Clear Sky if you haven't at least played Shadow of Chernobyl first.
I'll begin with my overall opinion of CS. It's inferior compared to SoC, and especially inferior compared to CoP. The Faction Wars system, one of the main selling points to CS, is confusing at best. You can side with three different factions in each area of the game (though you automatically side with one by default at the beginning), and then certain factions become hostile towards you. In return, you get the chance to earn extra jobs and top-tier equipment earlier than normal. The only problem is when you have to actually fight the opposing factions. You need to travel to certain areas of the map, take out any resistance you encounter there, and wait for backup to arrive. In addition, if any of those areas come under attack, you're expected to show up and help out. Thus, you sometimes wind up with scenarios where you're halfway across the map, and get a call from a lone survivor armed with a shotgun to high-tail it over to their position and kindly take out the group of seven guys all armed with military-grade automatic rifles, please. The worst part is that your opposing factions don't stay dead. I've captured every single point including the main base, and killed everyone I possibly could, and they just respawn the second you leave. The good news is, while you will still have to help your allies out, you still get paid a substantial amount of cash for taking out the main base, so it's not all bad, I suppose.
Without a doubt, the worst part of CS is the enemy AI. For the most part, it's the same as SoC, except for the fact that these enemies can use grenades. Even on lower difficulties, you'll be tapping the quicksave button every few kills just in case they decide to spontaneously send an entire squads' worth of grenades next to your feet. Even worse is the fact that the AI is ungodly accurate with those grenades. Wherever they throw them, they end up directly at your feet with a one-second fuse. It's not at all uncommon to watch an enemy throw a grenade in the wrong direction (as in, they're facing away from you), only to see it reverse directions in mid-air and teleport right next to you. This is especially bad on the final mission, where enemies literally spawn from nowhere armed with automatics loaded with armor-piercing rounds and grenades primed and ready. The most common thing I found myself looking for in CS was not ammo, or artifacts, or even money, as opposed to SoC. No, it was bandages and medkits to replenish any health I was losing from unfair grenades. If you're playing through on higher difficulties, your first priority is to find some armor that isn't completely terrible, and then stock up on as many healing items as you can. Otherwise, you most likely won't stand a chance.
If you're coming from SoC, you should know that the system for picking up artifacts is radically different than how it used to be. Where they used to be waiting on the ground in plain sight for you to just grab, they're now hidden in the centers of anomalies. You won't be able to grab an artifact unless you have your detector out (press the O button to pull the detector out, and press 2 to dual-weild it with your pistol), and unless the anomaly you're looking at happens to contain one. Most anomalies don't. Personally, I held off on artifact hunting until I got a better detector, but if you can deal with lots of trial-and-error and quicksaving, you're welcome to try.
Speaking of artifacts, not every piece of armor you find can actually equip them. For some, you need to take it to a person who can upgrade weapons and armor, and pray that they're capable of adding an artifact container or two. Thankfully, most armors can, although it will cost you a good amount of money, even more than upgrading a top-tier assault rifle. Speaking of upgrades, if you couldn't already tell, you can do that now. The same people who offer upgrades also repair your damaged weapons and armor (for a nominal fee, of course), sell you locations of stashes (which can also be found on dead bodies, although sometimes the person selling you a location will tell you what you can find before you open it), and offer you jobs which usually consist of "find information on a particular upgrade in a stash somewhere and bring it back to me". Keep in mind that you still need to find the stash first, as it's not marked on your map.
CS has a much different atmosphere compared to SoC and even CoP. While the other two are generally dark and dreary, Clear Sky is actually rather nice. By comparison, I mean. It's still the Zone, after all. It feels a bit out-of-place compared to the other two, but given that this is a prequel and things aren't as bad yet, it's to be expected. The mutants are still there, so it's not too different, but it is certainly noticeable.
All that being said, I still think Clear Sky is a fun game. If you can deal with the grenades, the different atmosphere, and the gameplay changes from SoC, you shouldn't have any problems at all with it. Be warned that it is a STALKER game, and is prone to bugs and crashes. I got lucky and only had two with this one, as opposed to CoP which wouldn't launch for the longest time, and SoC which crashed several times and wouldn't let me begin a new game without freezing until I messed with the options. If these happen to you, the very first things you should try are running Steam as administrator, disabling the Steam overlay while in-game, and switching the lighting options to Static. Those will solve a large amount of the issues you'll encounter.