What the previous posters said, and I'll add my five cents.

I've only played co-op with friends (because I would never play with people I don't actually know) and I can say that the game is super fun solo but even more fun with others.
That especially applies when something silly happens.

Solo feels more rewarding when you accomplish goals, since you do everything yourself.
On the flip side, you need to be more careful as there is no one to help if you, for example, die in a difficult spot.

Solo of course comes with greater freedom.
You also only need to get food and gear and such for yourself, which saves a -lot- of time and work compared to even just two players.