Short answer - yes!
Onyx is the best of the maintenance programs available for OS X. I know nothing of damage from too much use, but regular system cleaning helps the OS to run more smoothly, if not exactly "faster". Something like a sluggish browser is usually the result of an overload of caches.
By 'regular', that should be taken to mean as often as once a week for heavy use of your Mac, less often for lighter use. My eMac runs about 15 hours a day, most days, and I run Onyx about 2 or 3 times a month.
Which functions of Onyx to use?
1. Repair permissions
2. Run maintenance scripts
3. Update system optimization
4. Clear browser caches.
Others are more in the category of "do it if you want" One function I use is Onyx' capability to park screenshots where you want them, rather than on the desktop.