I have a Samsung 840 PRO 250gb SSD and when I bought it a month ago I got 557 MB/s read and 493 MB/s write which was great. Today I decided to do another benchmark and got 284 MB/s read and 266 MB/s write.
I am using iaStor and benchmarked both with AS SSD and Samsung Magician. The drive is neither half full nor the system drive. I recently reassembled my pc in a new case to fit my new gfx card and the good benchmark was before the reassembly.