I am testing SASS (and LESS) right now as well. Pre – processors have several benefits (which have been mentioned already) but they also have several drawbacks:
(1) You need to install and setup additional things on your computer. You need to do this on every computer you use for work. Plain CSS can be edited with even the most basic editor (e.g. Notepad) which means you can work on them from any computer even if you don’t have access to your own workstation.
(2) If you work with other people then EVERYONE should use the same pre – processor (this includes your clients if they might tweak the CSS code). CSS is a universal standard. SASS is not.
(3) Currently most editors do not support SASS. Those that do (most of them with plugins, which might not work when new versions of the editor are released) do so in a limited way. For example they might color code your SASS but code hinting and auto-complete might not work as good as with CSS (or they might not be available at all)
(4) Debugging your CSS code (e.g. with Firebug) becomes less direct and you might have to install yet more things on your computer to overcome this issue.
You have to weight the pros and cons and decide if pre processors are for you. For a website with 20 pages I doubt that SASS will actually help you. Then again, if you have some extra time available, you might try SASS out for a project just to learn the technology.
If the popularity of pre processors continues to increase then many of the above issues (editors, debugging etc) might be resolved in the next 1-2 years. At that time we might also know which pre processor will “win”, so you will not have to waste your time learning a pre processor that will soon die.
Finally, as somebody else said, if you are on Windows try LESS first. I found no good apps to compile SASS on windows, while i really like SimpLess for Less. Not only it compiles your code, but it also uses prefixr, which means you don’t need to use mixins to get cross-browser support. You just enter the standard CSS rules in your Less code and SimpLess will add vendor specific prefixes if and when needed.