In light of Apple having announced new iMacs today, I thought I'd throw out a data point to the internet.
tl;dr: I've used 2 DIY Fusion Drives on 2 different Macs for the past year, they're awesome, but you should use a Samsung SSD (I have an 830 and an 840 Pro).
Since last year's Fusion Drive announcement (and subsequent "reverse engineering"), I've been running 256MB SSD + 1TB HD Fusion drive setups on 2 Macs: a 2009 iMac (with the SSD replacing the optical drive) and a 2012 Mac Mini (using the extra bay after purchasing a second SATA cable). It's been awesome, especially after sorting the kinks out. (No, no benchmarks, but it really does feel / act as though it's an SSD-backed system, even when data needs to get shuffled around, as in the case of processing a couple hundred gigs of GeoTIFFs.)
The kinks: I'd already upgraded the iMac with a 256GB Crucial M4 about a year before, so I figured I would just back everything up and use that in combination with the Apple-provided drive. Bad plan. I went through 2 M4s (though probably not for the hardware problems they seemed to be manifesting) before throwing in the towel and buying a Samsung SSD 840 Pro, which has been completely problem-free since installing it. When I bought the Mini, I also bought a Samsung 830 at the same time, which has been trouble-free since the start (Apple uses, or used Samsung 830s at one point, so this seemed like the best choice).
With the M4s, large files (100GB+ VMware VMs) would spontaneously corrupt (presumably the filesystem passed the ~256GB threshold) and no amount of monkeying with Disk Utility could successfully repair it, ultimately necessitating Time Machine restores on multiple occasions. Things would be peachy for a week or 2 and then, bam, time to reinstall. No good. One Crucial rep I talked to when initiating an RMA suggested that the M4 wasn't getting a chance to run its garbage collection routines, probably due to the I/O patterns resulting from shifting data between disks.
Would I do it again? Definitely, though the move to PCIe SSDs complicates the matter a bit. In the case of the Mini, it was borderline unusable (by my standards, anyway) due to the I/O performance of the built-in 5400rpm drive, even with an i7 and lots of RAM. With the Fusion drive? Incredibly pleasant to use as my primary work machine for the last 9½ months.