I like FinderFileSize although I don't always have it on. I launch it via Quicksilver when I need it. The reason is that it sends a message to Console.app every 2 seconds!
When I do some debugging I don't want to scroll through endless pages of useless information like "2008-10-13 11:21:19.795 FinderFileSizes returns 9,93 MB [4]". I would prefer if it only sent a message when the value changes.

In case of multiple selected files I just wanted a tool which displays the size of all directly without clicking through menu(s) and without a new dialog to be shown. The menu bar is perfect for that but you are right that the menubar is full of other applications.

There is a similar app, which functions a bit different (it lives in your contextual-menu instead), called (freeware).

It's of course a matter of personal taste, but to me, CalculateSizeCM has some advantages over FinderFileSizes, as it's less obtrusive; you only get to see it once you right-click on your Finder selection, as compared to something that takes up valueable real estate in my (already) full menu bar. CalculateSizeCM can also distinguish how much memory is taken up by resource forks in the files and folders, which is super neato.