Personally I find it unbearable not to have the date and time in the menu bar, or at least somewhere on screen that I can see at a glance without having to click to see it.
For some reason the Mac usability guys decided that showing just the time, and date on one click, was more usable.
There are a lot of tutorials about how to customise this and generally each one has around 50 comments, the thing is none of them work without a black magic hack that can usually be discovered around comment #47.
If you don’t select the format medium and don’t include the seconds (even though they won’t show up) the formatting does not work.
I found this tutorial one of the more helpful ones, and after trial and error discovered the seconds problem.