SSDs really are good when it comes down to a machine under heavy I/O (Input/Output - Hard drive) load.
For example, the biggest improvement you will see is when you load Windows on a modern SSD.
Typically, it is hard to say the improvements you will see as it really depends on the application.
If you are talking about something like Photoshop, where it loads tons of external files and settings, you will probably see a huge improvement (at startup speed). However, if it is just a small, self contained application, it technically should be faster, however you are not going to notice as hard drives are fast anyway.