I wanted to get better performance from my mid-2011 Mac mini, especially to reduce boot times, as well as kill load times for games running under a Windows partition. As such I wanted to install a SSD into the mini, while keeping the the built-in drive for media and '2nd tier' data storage. The specs of these minis are good enough that I was noticing the built-in drive as a bottleneck to the system's performance, certainly when it came to boot & shut-down.
I paired the dual hard drive kit with a Samsung 830 256GB SSD. While I has the bottom off the mini I also decided to double the RAM to 8GB with a pair of sticks from Crucial during the install.
Firstly, the buying experience - second to none! If you're overseas and are worried, if my experience is anything to go by, don't be. The order was processed and packaged in half a day, and sent via Fed Ex international. It arrived very promptly a couple of days later across the pond, neatly packaged into a small box inside the Fed Ex bubble bag.
All the parts were in the box, and to my delight, the ribbon cable had been pre-bent and protected in its own small bubble-wrap bag, saving me about a page of (and the most confusing part of) the installation guide. The rest of the guide was very easy to follow, not rushing, I had the mini in bits in under 20 minutes - great instructions.
A small side-note on the Samsung SSD - thoroughly recommended. It has a great controller and capacity for the money, and is SATAIII like the Mac mini (it is also very thin).
I had this drive mounted in its bracket in no time. Re-assemble was a little more tricky - I had slight issues around re-mounting the built-in HDD, due to less space in the body for the power cables, and the cable from the new drive. This was by far the hardest part (just beating removal of the power supply cable from the logic board), but nothing a bit of teasing and tweaking wouldn't solve.
Overall, not a difficult operation at all, as long as you are able to be patient (removing the logic board feels a bit... wrong, tease cables in and out slowly and be careful not to over-stress them. The mini's internals are not like your regular tower for sure!
If you're looking for additional space, to create a RAID or, like me, install an SSD for faster drive performance, I can thoroughly recommend this kit and the associated guide.
I'm now writing whilst booted into OSX on the SSD - everything's 200% zippier and pretty much instantaneous. If only I could claim that my attempts to duplicate my windows install were as successful!
Also, the ifixit retail/support experience has been second to none. Perhaps the length of this write-up best demonstrates that.
Best of luck to all!