64GB really isn't sufficient for the average user to use as a Windows boot drive. If the drive manufacturer is using 64GB to mean 64x1000 and not 64x1024 you're going to lose 1.2GB. Windows 8 is going to take 20GB (assuming 64 bit). If you keep your page file on the SSD (required for memory dumps) you will lose the space equal to your RAM. Then you have temp files, programs that default to C: for everything, and data.
While it may be true it could become cramped, 64GB is enough, provided there's an extra drive for data & things such as games & virtual machines are ran from a separate drive. Probably could install a couple of games, if desired.
On my Toshiba notebook, am dual booting Windows 7 Pro & 8.1 Pro, plus have Windows 7's recovery partition on a 128GB Crucial M4 SSD, with almost 40% of it free. And plenty of apps installed on each OS. I do have System Restore disabled & the pagefile set at 512MB, with 8GB RAM, I don't see the need to have a large one.
Have a an identical setup on this MSI that I'm on at the moment, only it's an 180GB Intel 330 SSD, well over half of it is still free.
And on my Dell, as posted in my Speccy snapshot below, is barely using 30GB for Windows 7 Pro.
The main con that I see with a 64GB SSD, is that 120-128GB models costs just a few more dollars, like around $15-$25.