We're going on 6 years since it was released, and despite numerous patches and multiple hardware upgrades, the game still runs like I'm at about 70% of the minimum system requirements. It runs well at times, and then it will inevitably require a reboot to correct the memory leaks and general choppiness, and there doesn't seem to be any rhyme or reason to what triggers these issues in any given session. I could never quite get a handle on how best to run the game; every combination of settings and commands and tweaks I tried seemed to make little difference. I've never had this problem with any other game, certainly not for 5 or 6 years.
Many people in this thread are saying they had no trouble with IV, or only had trouble when they owned a low-spec PC, but this just proves the point even further - the optimisation is so bad that it only worked well on a very specific set of hardware configurations, a Holy Grail which most of us could never find. A well-optimised game would have the opposite effect, in that it would run well for most people, with only a few suffering persistent performance issues.
GFWL is marginally less intrusive and pig-headed than uPlay, but it's very much of the same needless, hateful ilk. The Rockstar Social Club was patched out of the game eventually, which was a pleasant surprise, but before that it was yet another cumbersome, unwieldy, and unwanted chunk of DRM injected into a game which was already dripping with it. The serial number, the online activation of a limited number of keys (and DEactivation! You couldn't even UNinstall the game without permission!) and being forced to keep the game disc in the drive to play it, all driven by the lovely SecuRom, wasn't enough apparently. On top of that, we had to install GFWL, create an account to use it, and then do the same with the Rockstar Social Club, handing over hefty chunks of personal information to two "services" no one in their right mind would opt into, and logging into both of these services at least once each before being allowed to play the game. To get from installing the game to actually seeing the main menu, you'd need to take the day off f*ckin' work. Pirates have to contend with precisely zero of these issues.
Playing IV takes real patience, it's an almost vocational pursuit. The enjoyment I get from the game is just barely ahead of the frustration I experience with both the performance issues and being treated like a despised criminal for having the temerity to actually pay for the game.
This is why I'm holding off on ordering GTA V. I know MP3 was well-optimised, but I've really had enough of making all of these personal compromises just to play a video game (admittedly uPlay has done the most damage to my mental health in that respect). If the reviewers start mentioning the same kind of sh*t that IV had, be it insane amounts of DRM and/or pervasive performance issues, I'm saving my cash.