In an effort to gain a little speed boost for my 2010 Mac Pro, I added a 488GB OWC Mercury Accelsior E2 as the boot drive. It has 2 SSD chips mounted on a PCI driver card which bypass the Pros built in HD nterface, so you can get speeds up to 688 MB/sec, much faster than if you simply installed an SSD in one of the drive bays.
As the title says, I'm very happy with the results. It really makes things much more snappy. AC launches very quickly, and there seem to be way less hangs than I used to see. I still have most of my files on conventional HDs due to the limited SSD size.
As a bonus, it has 2 eSATA ports, for attaching external storage.