- Regarding formal use:

It is a widely-held misconception (even among *some* young Japanese when they start working in a company) that you should use お疲れさま when leaving the office. Strictly speaking, this is incorrect.

お疲れさま[です/でした] is something you say **to people leaving**, or when you and the other person are **both** leaving (e.g. when you meet a colleague at the door etc.).

Saying お疲れさま[です/でした] when you are leaving, has a (very) slightly insulting overtone, in that you are essentially saying "I'm knackered and going home! [while you guys stay here]", which is not really the business Japanese way...

When **you** are leaving, it is probably more polite to say:

お先【さき】に失礼【しつれい】します: "I apologise for leaving before [you]"

Of course, there are also countless informal occasions where using any variations of お疲れ[さま][です/でした] is OK. 

Mainly:

   - at the end of any strenuous activity (or even not so strenuous ones, as a joke)
   - as a less formal and more "matey" replacement for 乾杯【かんぱい】("cheers", when toasting).

PS: I don't often hear お疲れさん in daily conversations: people will tend to use either お疲れさま (without です/でした → semi-formal) or just おつかれ (very informal).