I just paired my after-market internal 240GB SSD and external 2TB FW800 HDD to make a FusionDrive setup for my early 2008 iMac. I was running them as separate drives for about a year - my Application, OS, Library, etc. on the internal SSD, and my Home folder on my external FW800 HDD. In my case, the home-made Fusion Drive is actually faster than my previous setup of having separate SSD and HDD, as many frequently-accessed files (and I can only imagine things like caches, mail files, etc. as well) were on the external HDD and it slowed down my applications. Once I created the FusionDrive, reinstalled the OS and copied over my apps, settings, files, etc., I launched some of my more frequently-used applications. They noticeably sped up after launching them a few times - I could even see the individual disk activity in iStat Menus, and was able to confirm that after launching these apps a few times the SSD activity increased and the HDD activity decreased, indicating that key files/blocks associated with these programs.
If you would have asked me a couple of months ago if a FusionDrive had any place in my split-disk setup, I would have said "no" for sure. I was sure that I had things optimized by keeping the OS on the internal SSD and my documents on the external HDD. I am more than a bit surprised of the improvement that I have already seen in how well FusionDrive works. It will also make my backups easier, as I now only have to manage 1 drive versus 2 drives in my backup strategy. Unless you do major processor-intensive tasks, I'd say go for FusionDrive if you have to choose between one or the other. To be honest, even the "slower" processors of today are so much more faster than the "faster" processors of just a few years ago. As an example, my MacBook Air with its dual core 2.0 GHz i7 blows the doors off of my dual core 2.8HGz processor in my iMac.