I don't know that I'd call that a variation, it's really a different animal. Doing that will speed up boot and application launch, but the caching method has the potential to speed up any loading on the machine, which has the potential to provide much bigger speed gains. Wear and tear on the SSD is an issue, although that is lessened if they have a smart algorithm that puts more emphasis on caching most often used data and not writing everything that is loaded from HD.
That's going to be a very smart algorithm to be able to determine what is more important and what is less. I'm perceiving that it's going to work more like a big ram cache storing ALL of the most recent file activity with a "dumb" assumption that stuff you've used recently, you'll probably want to use again soon and stuff you haven't used recently, you'll probably NOT want to use again soon. Else, it's going to have to be able to judge mundane stuff like which website files should be cached and which should not, which iTunes file should be cached and which should not, and so on.
I'm also not so sure about the "bigger speed gains". In my own situation, big files (like applications) are permanently stored on the SSD and thus immediately available at being loaded into memory at full SSD speeds. Meanwhile little files like images & text in web pages I visit, doc files, music files, etc are loaded from the HDD. Sure, if those little files were also stored on the SSD, they would make it into memory faster than they would from the HDD, but they are so small, they are available to use really fast as loaded from the HDD too.
More simply, a fat application file on the SSD does load crazy fast compared to the same app loaded from the HDD. Bootup with those OS X files on the SSD is much faster than booting up via HDD too. BUT, as the files to be loaded become smaller, it's harder to notice any delay. Mathematically, a song in iTunes will get into memory faster from the SSD (or one of these cache SSDs (if I played the song recently enough) but perceptually, if the small song file is loaded from the HDD, there is not a noticeable delay that actually wastes my time.
Go very far down the alternative path and there would be no justifying an HDD vs. an SSD as one could argue everything should be on the SSD because every load will save some time. While true, that's also where you have to factor in cost of big HDD storage vs. equivalent SSD storage and the (too many) file writes problem of the SSD. Especially related to using one as a cache (for presumably all recently changed files to keep them nearly immediately available), I would think this would just use them up much faster than how I'm using the one that came with my iMac.
I love the idea of a fast cache. I just wonder about how fast such a solution would wear out without some super-intelligent algorithm to somewhat magically know what to keep on hand and what to ignore.
