It can work out nicely if most of what you do is the size of the SSD and the total capacity is higher than the SSD capacity (i.e. lots of rarely used stuff).

In fact, I worked on an enterprise storage that did just that and it significantly improved performance for many workloads. There are open-source solutions for that in Linux that integrate into LVM and do that pretty much without any other interaction by the user. bcache comes to mind but there were a few others as well.