Coolbits is pretty much just a registry file that makes certain changes to allow the Overclocking tab to appear in the Display Properties. Although sometimes the numbers that appear there are not the actual numbers. This is why I don't use Coolbits.
RivaTuner is a program that offers two ways to overclock. One being through regular driver settings (somewhat like Coolbits) but very accurate. The other way, is called "Low Level". What that means is that RivaTuner talks directly with the graphics card to set the OC level bypassing the drivers. Both ways getting the same performance from the card. Also, RivaTuner allows you to "fine tune" your DirectX and OpenGL modes for your card. Because of all these features, many people shy away because they don't understand the program. They opt for the easy way out and use Coolbits hehe.
Once you figure out how RivaTuner works, you'll love it. Guru3D.Com even has Forums dedicated to RivaTuner so people that have problems can find a solution.
i have used rivatuner.....it is an excellent program. All you really need to do is read the instructions to get it set up....it works just fine after. There are a ton of advanced settings that you can use if you want, but you dont have to.....
CoolBits is easier, but rivatuner is much more powerful.
If you want to tweak, then there's no question that Rivatuner is for you. If you just want basic overclocking, then you'd be fine with just Coolbits. I've always used Coolbits for my Nvidia cards. I like it because it's integrated with the drivers.