Rockstar has detailed out all the stuff they're working on to fix Grand Theft Auto Online. The biggest thing from Rockstar's update is that they've disabled the option of purchasable GTA$ cash packs.
Rockstar noted that the team is "working around the clock" to make the experience as smooth as possible. Until then, it's really up to luck if you can get in. Our own Chris Carter was able to play through GTA Online and make this handy tips guide.
As for me, I've been looking at the loading menu at the very first mission for the last 24 hours now.