Second update this year. FREENAS still runs non-stop and flawless. However with the reboot I noticed a hardware problem. FreeNas didn´t show my L2ARC cachs disc anymore. During a profound inspection I noticed that my OCZ Vertex II died. Of course it was not the newest SSD generation. And Caching files takes it toll on a SSD.
So my advice for a SSD as a caching disc: Look for a Samsung or Intel SSD of the latest generation.
Nevertheless, a caching SSD will suffer a lot more in a ZFS server than in a normal, average client pc.
However, it wil make your data transfer faster, and all your other discs will possible last longer.