The disc seems great and fast. However just a word of caution. The documentation that comes with the disc indicates that after switching the disc on it can take up to 10 hours to synchronise (RAID5) - blue flashing lights on the disc. Mine took 30 hours ! - Of course after 10 hours you start to wonder if something is wrong and after 24 hours even more so. Also the Disc utility software that comes with the disc (loaded onto it) might not work so you have to download the latest version from the Promise web site. This utility allows you to check the (very slow) progress of the synchronisation - it also has an option for you to try and speed it up.
Having plunged for a 3.4GHz i7 iMac with Thunderbolt and a 256Gb SSD I thought I'd need the best drive I could get for data such as my Aperture library and Video files. Well having come from having to use Firewire 800 as my fastest option before and getting around 80Mb/s on a 4 drive RAID 5 array and then getting a fantastic 200Mb/s on my SSD in the iMac I was curious to see any benefits. Well all I can say is WOW, it flies and makes the SSD drive look slow. On benchmarks I am indeed getting 500Mb/s "in RAID 5", not as I though would be the case, a RAID 0. The figures you see in the reviews of 500Mb/s for this drive are indeed the safe RAID 5 mode. If you want even faster go for the R6 which has 6 drives but you can easily upgrade the drives in this model at a later date. It takes any 3.5" drive and they come configured with Hitachi drives as standard although the R4 comes with SATA II drives and the R6 comes with SATA III drives. Not sure it really matters because I don't think a single 7200RPM drive will be capable of taking advantage SATA III anyway. 2Tb drives are around £70 so you could bump the capacity up to 8Tb fairly cheaply. Bear in mind, with RAID 5 you only get 3Tb of usable space and with 4 x 2Tb drives you would get 6Tb of free space.
If you want the fastest external drive available for a Thunderbolt Mac that's also safe with your data, don't hesitate.