Just a heads up for anyone with an older machine that only has TB1 and wants TB to PCIe capability: The Thundertek/PX enclosure is currently on sale for 139.95$. Price does not include shipping (+13.38$ for ground shipping in the US, no idea on international costs, but they do provide the option to ask for a shipping quote). Since the Akitio Thunder2 and the Rocketstor are 229$ and 234$ currently, this is significantly cheaper.
The obvious drawbacks are:
1) Thunderbolt cable not included (can be bought for ~20$ on ebay).
2) Only TB1 speeds.
3) The PCIe x4 slot is not open-ended, so a riser is required, or the rear of the slot needs to be modded.
4) Needs to be modded for use with dual-slot cards (but the same is true for the Rocketstor, and the Akitio in the case of long cards).
I am currently trying to resist buying one just to play around with it...