Here's little tour of the PVP environment of ARK: Survival Evolved. Things you'll face on these servers are harsh and you shouldn't go there before reading this, especially if you haven't even played the game yet.
Accepting the rumors
The things that people say about the griefers is true: They will kill your dinosaurs, steal your stuff, abuse your body, and destroy your house as soon as you go to sleep. In fact, a lot of tribes organize a group to play past midnight for the sole reason of hunting sleeping players. And that's the keyword here: Tribes. If you are playing on a PvP server, you have to keep in mind that you're not supposed to do it at your pace and take your time building your little tatch hut with it's little thatch fence with your raptor and a couple of dilos. Slowly learning the game by yourself, taking everything in, eventually upgrading to wood, then stone and so forth.
If you plan on playing in a PvP server, you gotta take this game seriously. You either find a group of friends to play with you, or you join a tribe as soon as possible. If you play alone, you might find a hidden spot where your stuff will be safe for one or two nights, maybe even a week, but alone, you're doomed to be found, robbed and killed. You'll have the work of a whole week go down the drain. If you're joining a PvP server, you're doing it for tribe warfare and that thing can get really intense pretty fast. With eagles swooping down to grab enemies and dropping them from high altitudes to their deaths, or tactical dropping raptors inside their base to wreak havoc, T-rexes working as battering rams at the enemies' gates while rockets fly at them, auto-turrets spewing bullets at your carno army, sabretooths leaping over walls. It is a beauty to behold.
But you'll only experience that if you join one of the dominating tribes on your server or work on creating a strong tribe of your own. Otherwise you'll just be cattle for them, nothing else. They'll keep you from taming dinos by killing them during the process, block caverns so only they can harvest its resources, literally kidnap you to use you as a blood bag Mad Max style and so on.
It's understandable why so many people hate that. After all, they're just being preyed upon, they're not participating in all the fun that is Tribe Wars. But the only way that makes Tribe Wars so much fun, is allowing this kind of freedom the game has right now. They gotta allow people to be complete jerks, because during wars, that's exactly what you have to be. If they put too many safety nets to keep people who wanna do PvP, only when THEY want to play PvP, the game would be boring and stale.
Still worth buying
If everything mentioned here right now put you off, don't be, because you don't really need PvP for this game to be fun. There's a ton of stuff to do in PvE servers or offline if you just wanna do everything at your pace. People are also very helpful on PvE servers. They'll give you a hand when you have problems with a rex, help you gather prime meat when you're taming a carnivore, give you tips and materials.
The developers are doing a terrific job. Optimization is great, the content is fantastic and bugs are rarer and rarer. Ark is a childhood fantasy come true. It's a mix of Jurassic Park and Minecraft with a touch of DayZ. Ark: Survival Evolved is the kind of game where if you ask for help or report a glitch on the forums, you will be answered by an actual developer and the glitch will be addressed within hours to days. Wildcard Studios "from day one to the present" have been very responsive and are an excellent team.