GTA V is still worth buying if you have friends to play with. GTA Online is heavily featured on co-op. The heists are great, it's a pity there's only five of them; but there's a lot of replayability there. The two recent DLCs, namely Executives and Bikers, are also heavily reliant on co-op.
Don't buy it if you're going to play it with randoms. Most of them are a nightmare to play with and will ruin your fun and your day. This goes doubly in free-roam. There's a whole slew of 12 year-olds, Chinese hackers, and 12 year-old Chinese hackers in GTA Online whose version of fun is to ruin others'.
This doesn't mean that GTA Online isn't fun. With the right players (or friends) it's loads of fun. There's something to do for everyone; you can race cars, bikes, planes, helis, boats, you can go on heists, you can do missions, you can have a biker gang (Sons of Anarchy style) and operate several illegal businesses, you can be a CEO of a small organization. I have sunk about 850 hours total into it and it's worth the price. Be forewarned though, to be able to afford most of the stuff in online you'll have to grind a lot. So if grinding isn't your thing then don't get the game.