Not sure about the single player, not played enough of it to give an opinion.
The MP is good fun in my opinion. The maps feel to small for 64 players though, for conquest anyway, but still fun. I've not tried other game modes yet so can't comment on those.
The way I see it is, BF3 is old and a bit boring now, BF4 is total shit and i'm done with it

So Hardline is something to fill the online FPS gap till something better comes along.
You can get it for around £25 on places like G2A, unlike the £50 Origin wants you to pay for it. For £25 I'd say it is worth it, especially if you are bored with BF3 and fed up with BF4 and you want something new to play that isn't buggy and broken.