I just got done "taste-testing" all the OS X VNC viewer (client) apps I could find, and this one gets my vote. VNCThing and VNCDimension seemed very nice also, but there's no way to get rid of the scrollbars in those viewers. I have my VNC server (on Solaris) set up so that it fits exactly on my Mac screen in "normal" (non-fullscreen) mode. Hence, no need for scrollbars. If I don't need them, I don't want to see them.

I like the fact that COTVNC is hosted on sf.net. This implies (but doesn't guarantee :) that the app will be supported in the future.

I don't like the way buttons 2 and 3 are emulated in COTVNC, but I'll deal with it. :)

I don't like that Ctrl-Tab doesn't get sent through to the server, but I'll deal with it.

I like fullscreen mode for when I need to connect to a server which has dimensions that are larger than my Mac's display.

COTVNC seems to be as responsive as the other 2 I mentioned above.

And I like the price.