GTA 5 went live about twenty minutes ago, but already an unexpected problem has emerged. Some users are seeing the message "the Rockstar update service is unavailable (code 1)" when trying to install the game, or else the launcher is crashing to desktop. Rockstar Support has already released a workaround for the issue, which is caused by Windows usernames with unconventional characters.
Basically, if the Windows username you're using features any characters not included in this handy list, then you'll need to sign in with another user account, or else create a new one. It's only a temporary solution – Rockstar is working on a fix as we speak – but if you can't wait to get started, it's good enough.
Simply changing your Windows username won't work, Rockstar reports, and if you do choose to use a different account in the meantime, you're advised to restart your PC before continuing.
If you are in the game already, why not share your benchmarks and screenshots?
Update: Chris is reviewing GTA for us, and has had problems launching the game that are unrelated to the above Windows username error. Specifically, Steam displayed an 'Installing' window for a few seconds and then closed, after which the game wouldn't load. Fortunately, he's found a possible fix.
Open GTA 5's install directory, found by right-clicking the game in the Steam library, going to 'Properties', and clicking 'Browse Local Files' in the Local Files tab. From there, go into the 'Installers' folder and manually install the Rockstar Social Club through the installers found in that directory.
From there, it should load normally—although we think some of the problems are due to a Rockstar Social Club connection issue.
Update 2: If you're having any other issues, check out our more detailed GTA 5 PC fix list.