You may think having an application use your processing time would slow down your system, but Folding@Home was built to be extra-considerate. It uses the lowest possible processing priority, so whatever other task your computer has to perform is considered more important. In my testing, I could feel no slowdown at all, despite the Folding@Home client utilizing 41-46% of my CPU time while I was working.
Since Folding@Home does make use of your CPU, it can cause some increase in power consumption. When your computer is running on main power, this should not be a concern. It may become an issue when you're working on a laptop in battery mode and every extra minute of battery life counts. Fortunately, the client can be configured to pause work while on battery power. Note that this is not enabled by default, and the client will keep computing even while on battery power if you don't set it to pause.
As a client, this is not a very exciting application: You can get it to display a cool-looking 3D molecule on your screen, and that's about it. The excitement comes later, when medical advances fend off life-threatening diseases--and you know that you helped find the cure.