My opinion after playing the single-player mode for 6 hours:
First of all, I would like to thank @Eos for giving me both The Forest and Ark: Survival Evolved as part of his very generous giveaway.
I've played the single player of The Forest for the last 6+ hours and wanted to share my personal take on it. I'll begin by saying that the first 6+ hours in the single player version of The Forest was the most fun experience I've ever had in a survival game, and one of the best experiences I've ever had in video games at all. I didn't even tried the multiplayer yet because I couldn't get myself to stop playing the single player. I took a break for now only to post here about how much I enjoyed playing so far. While playing, I joined Discord and told my friends about the game, and they decided to purchase it and really enjoyed it as well. We all played the single player version while being together in Discord, and although we weren't playing together, sharing our experiences, reactions and helping each other out was really enjoyable.
I will begin by saying that this is the only survival game that managed to make me feel scared. I'm personally not into horror games, but for some reason this was my favorite part about this game. I really enjoyed being scared, and it just shows how good and immersive this game is. When I got used to a certain threat that managed to scared me, the game always managed to send a bigger threat to scare me again, making the single player experience intense.
Another thing I really love about this game the crafting system. In most games I've played so far, the crafting system was always one of my most hated features, but The Forest really managed to make the crafting system one of my favorite things about the game. I like everything about the crafting system: the book which includes many crafting recopies, the fact that there are still some recipes left for us to discover, the way building works in this crafting system and the fact that you can both use the structures that the game offers you or get creative and make your own.
In addition, the resource managing system (hunger, thirst), which can be a pain when it is not done well, was really well done, the graphics are really good in my opinion, the game runs smoothly on my PC and it has a co-op and multiplayer modes which I haven't tried yet, but will probably be as good as the single player mode if not even better. I've also noticed that this game gets frequent updates, which is another plus.
I cannot really think about something I didn't like about this game after playing it for 6 hours. I hope that it won't get repetitive the more I play - but the game is still in Early Access and gets frequent content updates, and there is also the co-op and multiplayer mode that I still haven't tried. I'll edit my review once I'll try the co-op and multiplayer modes along with my friends, but so far I really enjoyed this game and really recommend you to get it even if you aren't fans of survival games. It currently costs 12$ on Steam as part of the Summer Sale and will keep being on sale for the next 40 hours.

My opinion after playing the co-op and multiplayer modes along with my friends:
Coming soon.

Again, thank you very much @Eos ! You really made my week :)