To summarize, here are the pros and cons of adding an SSD:
PROS
===
SSDs are 10x faster than traditional hard drives, making a **dramatic** difference in the speed of all read and write operations (which is a big chunk of what a computer does, so an all-around feeling of swiftness, particularly on bootup, program opening, and saving).
SSDs have no moving parts, so are less likely to fail *due to trauma* and may be less likely to fail generally than spinning HDs. They also use less power and run cooler (again that no moving parts thing)
Thus, they are a very cost-effective upgrade for an older Mac in good shape, up to a point (my personal opinion, Intel Macs -- you can probably add certain kinds of SSDs to PPC machines, and I'm sure it would be dramatic, but those machines have other issues that make it "good money after bad" at this point).
CONS
===
SSDs are more expensive than spinning HDs, which is why traditional HDs make great backup drives, where speed is not the highest priority. Although the price of SSDs has fallen a great deal, so has the price of HDs. SSDs are still around 4x-5x the price of a equivalent HD, but the performance difference is generally worth it. I prefer SSDs for internal drives and USB 3.0 (if your hardware supports that) external HDs for things like backup drives and media drives, etc.
SSDs tend to give fewer (if any) "warning signs" before they fail; they tend to fail suddenly, albeit rarely. Thus, backups are still a must, but your chances of the drive being okay after a fall for example are higher I believe. They're not shockPROOF, but they are more shock-RESISTANT.
Should an SSD fail, it is much more difficult to retrieve data than it would be from an HD.
I hope that helps you.