** MID November 2014 Update **
Going into Mid-November and onto the holidays, Apple/Samsung SSD's continue to offer the best value for cMP upgrades to PCIe flash. Prices for the 1TB SSD hover between 500-600ish, and offer comparable performance to the $520 M.2/NGFF 512GB Samsung XP941.
Apple PCIe adapter $10 on ebay
The Snitech Apple PCIe SSD adapter is
currently available on eBay for $10. While there is a newer revision with an unnecessary heat-synch, save your money and buy revision one.
Apple SSD Deals continue
Apple 512GB SSD's pulled from rMBP's have been seen on Ebay for $250(I bought one) and 256GB for $125 and offer great x2 performance at stellar prices. (this was last weekend). If you are in the market for an Apple/Samsung SSD, save some money be setting up a product watch on Ebay and buy when the deals show up.
The Fastest SSD - Ever
There have been two lucky ones purchasing 1TB Apple/Samsung SSD's on Ebay that have won the SSD Lottery! A new generation of NVMe based SSD's with 1.4 GB/s performance has started to show up prior to retail availability in any Apple product. Pushing the Classic Mac Pro as a "Benchmark" for the nMP to attain. (well once they have the SSD's.. lol) for now, we have the fastest consumer oriented SSD's available in the market... Sweet!!
What should I buy?
These day's it's hard to take a pass on Apple/Samsung SSD's for the cMP. With Adapters at $10, and great prices on gently used products for the low end, it offers a Trim ready plug and play solution to bring faster than SATA3 performance for the cMP for 128, 256 and 512GB variants. It's hard to recommend a XP941 512Gb. For <$100 more, you can jump to 1TB. The XP941 does hold the performance crown over Apple's 128/256 and 512GB SSD's, bit a hefty price increase.