... a few days later...
Non compressible data is exactly as it sounds- data that can't be compressed. Usually large media files, video game files, etc.
Writing is usually slower, because flash drive controller employs a special writing technique. The drive usually just writes to the next available free space on the drive and doesn't write to the space previously occupied, like normal drives do. It sounds like this shouldn't be a problem, but the drive also needs to mark the data that's going to be over ridden to be actually deleted when the drive garbage collects.
So the data isn't actually deleted, until the drive is full. When the drive is full, the garbage collect process occurs and purges the data marked for deletion.
If the controller did not do this, and just overrode the data like hard drives do, the controller would be significantly limited in its ability to put out IOPS (basically a usability issue) and it would wear physical cells on that space so cells would wear prematurely.
In my experience... well I have basically 3 generations of SSDs. I noticed the biggest boost from the Agility to the Vertex 2, which is like Going from the Toshiba to Samsung for the last gen Macbook Air. The speed difference between a Vertex 2 and Agility 3 was much less noticeable, which is like going from a Toshiba to Samsung in the current gen Macbooks. It's probably not a good idea to compare different SSD brands, especially because Samsung uses their own controller but it should give you a rough idea of the performance difference.
