You may read about glowing benchmarks for the newest SSD's.
The sequential benchmarks drive the SSD to it's maximum with programs that issue I/O operations
at a much faster rate than an application can, and does so at high queue levels. 6gb sata looks great.
But, a normal desktop user rarely does anything remotely like that.
The second type of benchmark measures maximum IOPS which will be done at high queue levels again. Think >30.
That is also not what we do. The OS does mostly small random I/O, and at smallish queue lengths.
It is the response time that matters most.
It turns out that at low queue lengths, Most SSD's have the same response time, and they are very low.
That is exactly what you want from a SSD, particularly for the OS.
So, what does this mean when buying a SSD?
Buy the capacity you need from a vendor you trust.
Today, I think Intel and Samsung seem to be the most issue free.
Also, buy enough capacity so that you will have sufficient free space to maintain performance.
Have perhaps only 75% maximum usage.