So really, the only DOWNSIDE is SSD is it costs an arm-and-a-leg to get from Apple on the iMac. Or it's a REAL PitA to install on an iMac.
The problem with Apple's SSD solution at the moment is only lack of options (there are no 256GB or 512GB configurations).
As for the price, considering either educational or EPP discount, Apple gets close to $1 per GB, similar to 840 Pro. Also, you don't need to buy the "kit" and do the cracking yourself.