Singleplayer is a fantastic game in its own right. It's fun, but it also does a good job of parodying and highlighting American culture.
Online can be incredibly fun or boring as hell. It depends on what you want to do with it and how you deal with other people. I have over 300 hours in GTA Online and still play it regularly. The game modes are fun with a few exceptions and racing being my favorite. The community overall has improved over time I'd say as most players really enjoy the game and the amount of kids trolling everyone has dropped off a lot. There's still assholes, but they're the minority anymore. Hacking is there of course, but it has decreased significantly since PC launch. It's somehwat common to run across hackers spawning money and being friendly or spawning campfires inside your body that persists through different sessions and burns you alive... That was annoying. Also seen things like removing all your guns or attaching a plane to your forehead. Passive mode lets you ignore most everybody if they're being hostile though and as I said this is less common now.
The biggest problem with Online is the economy is heavily inflated to encourage purchasing ingame money. Stuff is quite expensive and it can feel quite grindy when you're getting started out. Missions and heists get you good money though and if you have people to play with this isn't as much of a problem. I find the jobs themselves fun so it doesn't bother me to do them a lot. I have 5 garages, 4 full of customized cars. The frequency of money spawning modders also helps this, as one of my friends spawned me a bunch of money and eventually got banned. Lol.
Also if any of you ever want somebody to play with, feel free to add me on Steam. Name is Denther.