It's fun.
I was a bit frustrated at first. There's a mechanic that allows your NPC bots to revive you when you die, and I thought this was too co-op focused and made the game kinda trivial, but it turns out this is quite limited to like 2 revives and then you die for good, so it's not that bad.
Aside from that, there's some sections that are way harder because of the bots, but they are doable solo. Just keep in mind that Veteran (hard) difficulty can get quite hard because of this.
Aside from those 2 things, the game is perfectly playable solo and very fun.