They're great for for single drive systems where you don't want to choose between an SSD and an HDD; these hybrids only have 8gb of SSD storage on them, but that's plenty for speeding up boot-times without sacrificing capacity, which is perfect for laptops or other compact machines like HTPC's, which is really what they're intended for.
The caches should work for non startup related files, but the OS will take the biggest chunk so it's unlikely you'll see much of the SSD like speeds unless you use it with a system that's running continuously with infrequent shutdowns/restarts, in which case more of the cache will become related to what the machine runs most often, it just takes a while for that to happen.
I've often wondered what it'd be like to use these in things like RAID setups, but it's not really what they're for. It'll make a great drive for extra capacity if you decide you need some, but personally I've found that an SSD + HDD Fusion Drive setup (or self managed if you can be bothered) is vastly superior to one of these drives, as it'll keep most OS files cached plus other frequently used apps and files.
So yeah, great for smaller single-drive machines, or self-builds on a budget, but probably not so useful for your setup.