The OWC Mercury Accelsior PCIe SSD is my all-time favorite SSD for its blazing speed. A single Accelsior is faster than an 6GD conventional SSD, but I actually use dual 960GB Accelsiors in a RAID-0 stripe inside my Mac Pro (for capacity and the ultimate in speed for big files, well over 1000 MB/sec).
The Mac Pro accepts a PCIe card internally, but any Mac with Thunderbolt can use a PCIe SSD via the OWC Mercury Helios enclosure.
Update: OWC has pulled references to the Accelsior 'E2' model, so I inquired— the Accelsior E2 card is in development and there will be a relatively low cost upgrade option for existing Accelsior owners! Essentially it will be a flash module swap onto a new board, the return of the old board netting the cost out to a very reasonable level.
Anyone with legacy hard drives that include eSATA ports (but perhaps not USB3) will find appeal in this arrangement.
Because my Mac Pro’s PCIe slots are full (dual Accelsiors plus an eSATA card), an Accelsior E2 holds a lot of appeal; two of them together would give me the same SSD capacity but also give me four eSATA ports. Then I could free up the slot that is currently occupied by the eSATA card.
* NOTE: OWC’s site lists the Accelsior E2 availability as “30 days” but this is uncertain; official word on shipping not yet know. Also, the site currently shows the Accelsior E2 only bundled with the OWC Mercury Helios enclosure.