Just to clarify, there is no SATA 6 revision or "standard".
The current X-25E does not support the SATA 3 (6 Gb/sec) speeds. So your SATA 3 motherboard/southbridge would negociate a SATA 2 connection (3 Gb/sec) with the X-25E.
Various tech websites continue to show the X-25E as one of the fastest SSDs available, particularly under a heavy random workload.
So before making the plunge, I recommend you understand your workload. If you have a highly sequential workload (continuously movie editing or creating large image files constantly, for example), it may make sense for you to wait for a SATA 3 SSD. But under a typical client workload, it's doubtful that you would experience a noticeable difference in performance using a SATA 2 or SATA 3 SSD.
Also, if this SSD is for use in your home or work PC, an X-25E may be overkill, since it is intended for extreme usage situations like in servers. You may get better value with an MLC X-25M G2.
Hope that helps.