Depends a lot on what ssd if it's going to perform better in some situations. Like some ssd have crap write speeds (where I am calling anything under 200mb/s 'crap' and which is even more true on sata 3gbps) that are about the same as a fast hdd, so clearly it wouldn't give much benefit there.
Lately I have been using ssd for transcoding, then copy over to the hdd when it's done. I can see a definite improvement there as my ssd has 5x faster writes than my hdd, so it actually does finish about 5x faster since it's not being held up by write performance. While I don't do much things like 3D renders, I am pretty sure the same can apply there if the rest of the hardware is performing the task fast enough where it's being held up by write speeds.
Now is not a great time to buy ssd really to invest in 'top performance' sata ssd currently on the market... there is too many new things coming. Most important of which will be sane priced consumer level pci-e based or m.2 ssd using the new nvm express interface which will become new common standard. NVMe has a massive performance increase, and will leave current sata ahci ssd in the dust. Pairing NVMe with new 3D nand, and we may end up with drives that perform magnitudes faster than sata ssd at cheaper prices.