I currently have a C2D 2009 MBP 17'' with a stock apple 256 GB SSD (toshiba).
With this hard drive, I feel SEVERELY limited, I have to store a lot of my dataon an external USB2 drive of 1TB, which I also use for time machine backups, and I had to buy a 48GB ExpressCard SSD for my VMs (FileMate SolidGO ExpressCard 34 Ultra).
To give you an idea, it's about 1 year I've been hovering with less than 10GB free on my drive, many times less than 5, and each time I want to download/install something I have to delete something else. And this situation was with full optimization of the drive space like using utilities to cut out languages/architectures or remove printer drivers, etc...
I pondered if going for the 512GB or upgrading to 768GB, in the end I felt so tight on the 256GB on my current MBP for so long, that I didn't want to incur into the same limitations soon again on the rMBP, so I went for the 2.7/16/768 ultimate.
Heck, If they gave me a 1TB upgrade for another 500$ on top of the 768GB price, I would probably have taken it. I think the Samsung 830 768GB is definitely worth that money.
These 768GB I hope will allow me to be able to store everything I need (iPhoto, VMs, etc..) on my notebook, and maybe even be able to cut a 100GB bootcamp partition for native window gaming, and not be forced to plug in the external drive every time I use the computer, which is something I loathe. I will have to plug it in only a couple of times a week for time machine backups.
Overall given the fact that I've been severely limited by the 256GB for years, and I have on top of that another 50-100GB of VMs, plus I plan to install several 10-20GB games on OSX side (WoW, Diablo 3, Deus Ex, The Witcher, etc..), and I plan to have a 100GB bootcamp partition (100GB is enough for no more than 5-6 games installed)... in the end I think the 768GB will be just 'enough', and 1 TB would be needed not to feel any 'limitation' at all.
This is my situation.