I just bought a nearly maxed out 27inch haswell iMac but I only opted for the 256GB drive. I have a MBA which only has a 128GB drive and I still have 50GB left. Probably because I have all my music in the cloud, stream my movies from iTunes or Netflix and my photos live on an external drive. I'm in a better position with this iMac as USB 3 is only about 10% slower than internal sata and I prefer to dump all the large files on external disks. What is taking up all your space, as I am questioning whether 256GB is enough. It's only a small doubt in an otherwise awesome prospect of having a SSD main drive and USB 3 drives that are almost as fast as internal storage.