Comes with an SSD build-to-order option, or a fusion drive option, which is a SSD + large HDD with the SSD as a block-level cache.[1]
If it's the same as last year's model[2], it isn't that hard if you are used to working on laptops or mini-itx cases. Just have to be careful about dust, have a few torx bit screwdrivers, and some adhesive strips which you can find for $15-20.
I already have 3 SSDs floating around that I know are good (Samsung 840 Pro). I wouldn't want to kick out for another 256Gb SSD when I already have one. Not only that, those fusion drives are terribly unreliable from experience.
Regardless of that, that process is HELL compared to another machine. The thing I'm typing this on now (Lenovo T400) requires any old screwdriver you have lying around and 2 mins.
1960s point to point wired televisions are easier to repair.
I wouldn't want to kick out for another 256Gb SSD when I already have one.
All current Macs[1] currently use PCIe SSD drives, which are not interchangeable with SATA SSD drives. They are, however, around 10x faster than SATA SSD drives, which is something people never seem to mention when comparing 'specs'.
[1] with the exception of the 13-inch non-Retina MBP, which is presumably being phased out.
If it's the same as last year's model[2], it isn't that hard if you are used to working on laptops or mini-itx cases. Just have to be careful about dust, have a few torx bit screwdrivers, and some adhesive strips which you can find for $15-20.
[1] http://en.wikipedia.org/wiki/Fusion_Drive#Design [2] https://www.ifixit.com/Teardown/iMac+Intel+21.5-Inch+EMC+263...