When you are connected to the computer via VNC, you are able to control pretty much everything on the screen, with the exception of special keys (i.e. command, option, F keys, etc.). You can, however, "pinch" the iPhone's screen to zoom in or out the viewing area.
Now for the gripes. Mocha VNC Lite uses left-clicks on the screen (which can be quite annoying after a few minutes use). You are also unable to change the refresh rate (however, if you click the "+" button, you're given a "refresh option.")
Mocha VNC also offers a paid version which gives you more useful features like extra PC keys, cursor keys, text macros, and a right mouse key. However, the lite version will give most users everything they need for casual VNC management.
You can download Mocha VNC Lite from the App Store for free. At the time of this writing the paid version has not yet been placed on the App Store. Feel free to look at our gallery of screenshots before you download.