To be honest, It's very much playable for me. I have clocked around 6-7 hours in total & completed 18% of the game. If you look at the numerical stats, they are very uptight. I play at around 1280x1024 resolution with Texture Quality set to High & Render quality set to Very High. The rest i.e. View distance, Vehicle Density, Detail Quality & Shadow detail is set to 30, 15, 40 & 0 respectively. I get around 22 fps while cruising normally on foot & drops to 15-17fps while driving at faster speeds on a vehicle.
The game doesn't scale at all. Lower the settings to any amount & you'll barely get a large frame increase. But it doesn't feel laggy meaning the visual lag is barely visible. Just like Crysis, this game doesn't feel like it's being dragged even if it's at 17fps. But if you're a numerical & FRAPS whore then yes it would be wise to stay away from it as it won't be a beauty. Since you have a quad processor, you'll definitely get frames above 30'ish (or atleast hope to get above it). All you need to do is get those tweaks in place.
My advice is forget about it's performance & get the game. If you have a config similar to mine (in this case better than mine) then you'll seriously enjoy the game albeit it's visual flaws. Also cross your fingers & hope the patch Cockstar is about to release is good enough.