GTA V has just been released all around the world and is available for anyone to buy - providing you can find a place which has stock.
However every game has its teething problems and the Xbox 360 version has one that doesn't affect actual gameplay, but more the graphics and experience.
The Xbox 360 version comes in two discs, one disc required for mandatory installation and the other for playing the game. You can install both discs on the system but Rockstar are advising against it. If you install both discs you end up getting some noticeable pop-in issues.
It looks as though the problem lies in the Xbox 360 itself where it's finding it difficult to load so much data at once. So you are much better off doing the mandatory install, then running the game off the other disc instead of installing it.
PS3 users don't have to worry about this issue because of course Blu-Ray discs can hold much more data. But Xbox 360 owners should follow this if they want the best experience.