All dock programs (including the original on on OSX) causes your CPU usage to spike when you're hovering the mouse over the dock, however, I don't notice an actual slowdows. I like to keep, my taskbar as clean as possible so I stopped using the quicklaunch and I really don't like the limitations of the startmenu (requires too many clicks to get to wat I need compared to a dock) Plus the docks don't show up when you're in other programs. It's just hidden until u move the mouse to where you put it (in my case the bottom of the screen) and disappears when you click back into the program you were working on (normal z-ordering with mouse activation for those who know what I'm talking about)
For objectdock the usefulness of it comes down to 2 words: Fly-out Menus. These are a godsend for me. It allows me to put very few icons in my dock and still have access to every program I use.

FYI: The first icon on the left functions as a start menu, oh and Odock only uses ~1-4mbs of ram.