I only play single player. The game has been good for me so far. You should be able to get a couple of hundred hours out of the game in it's current state. Then get some more hours as more content (biomes) is released.

yep its really great and best of all if you can jump into other peoples games or they can come and help you ;-)
I think the game is great both in single and multi but i rpefer single player since it can be a bit annoying if someone is not on your "level" of technology in game and it can be comfusing when someone ahve something you have not unlocked and so on.. But els great we can build together, share resources, and fight sogether ect ect.
I can highly recommend the game its really great.

Refund and get the game for free instead if your playing solo

it's so expendable with mod support, and have high potential. i would keep it.

It is very doable and still relaxing solo and you don't have to worry about draining resources if others aren't gathering as much as you are. However, it can be more fun to sail together or pick apart enemies with someone watching your back.

absolutely. I have played a lot solo

Playing Solo. Just before the Third Boss, almost end-game swamp. I've only been playing for a couple of days, I'm not sure where some complaints about difficulty come from. I really like it, super intense and immersive solo.

Very doable solo. Just remember this isn't a mindless run in blindly hack n slash, that sort if behavior will get you killed, frequently. Planning, prep and caution are important.
Also you don't have to fully upgrade gear level. If you're good at combat you can use the first level of gear for each biome just fine and save a lot of time farming mats. However if you are struggling you can invest time in upgrading your gear levels to make things a lot easier. The choice is yours.