Is Singleplayer good?

Sins singleplayer has no story. It's kinda the sandbox you get in games like Civilization. (I.E. you have a win condition and the gameplay.)

Plenty of people never play online, though these forums being what they are, someone is sure to try and convince you to do so.

If you have no problem with sandbox strategy games yes, you will like it. The Sins AI is much better than in other more popular games, the only reason it gets flak IMO is people who either don't like what little, subtle story there is and want a campaign to explain it. The only real problem is things like a real lack of game modes and other interesting settings, but this is to be somewhat addressed in Rebellion. Also if the game does get boring there are lots of mods to make it interesting again, which if you are a fan of watching space battles is worth it just for that.

All and all, if what you've heard about Sins appeals to you so far, I doubt the single player is going to change your mind.

I play exclusively SP and have for years.  I thoroughly enjoy it, though over time I've become progressively more adept at modding, so now I run custom personal mods when playing most of the time, I still play the default/vanilla from time to time.

SP really is wonderfully entertaining.

SP is very enjoyable and the AI difficulty levels allows for scaling of ability as you learn and become better at the game.

I mostly play single player, but nothing is better then the occasional get on with a group of people you know from the forums and challenge PBhead to a match and watch in horror as he kicks the living hell out of all nine of you.  I mean....  Uh...  Um....  I'll go back to modding now.