I finished GTA 4 this week. I played about 2/3rds of it in the first fortnight after release, then got really frustrated with some of the missions where I kept dying and wasting hours regularly and I'd had enough. I stopped playing. I managed to speed through it from pretty early in the game to the end using cheats and avoiding the social call things and I guess I...liked it?
I really don't like the awkward driving, shooting and cover systems, whilst the incessant social calls and lack of checkpointing actively made me quit the game. With that said I really liked the story and the characters, and some of the mission design was inventive, even if some of it wasn't. Some ups, some downs.
Does the DLC make much improvement with checkpoints and social stuff, and still have good story and characters? If so I might give them a shot as I own them already. I probably won't get around to it right away as I have plenty of stuff to play, but I'm more thinking about whether I should just uninstall the game completely or leave it hanging around and I'll play the rest later this year maybe.