Yeah definitely worth doing.
The SATA II will be a bottleneck, BUT ultimately the extra speed you gain from an SSD is the random read/writes which ultimately don't tend to be faster than SATA II can handle anyway. The only time it'll be slower than SATA III really is during sequential read/writes of massive files. I think that even if you added a SATA III card to your PC and tried the SSD on there to compare, you wouldn't see a proper real world difference
EDIT: Take a look at the below article, you can see that in some situations, the SATA III interface makes a fairly big difference in the real world tests, for example, copying a 16GB file. But the SATA II SSD is still significantly faster than a mechanical drive.
The interesting part is the "boot times" real world benchmark, the interface made practically no difference at all, and looking at how each SSD performed, the only difference was how fast the SSD was (ie, the Crucial M4 on the SATA III interface was first to boot, but the second item on the list isn't a different SSD on the SATA III interface, it was actually the Crucial M4 again, but on the SATA II interface, showing that having a better SSD is actually more important at boot time than having a better interface).
I'd love to have a go at booting up an SSD on a SATA I interface to see what would happen!