It's a pity/shame/bummer you are leaving us! I wish you all the best in your new endeavor/position.

AE

Note that "bummer" is a slang term. Not a good idea to use it unless you understand the connotations/resonances of the word. It sounds like the way an aging California hippie or surfer-dude might speak.

thanks!

eric, #2, I wouldn't consider bummer neither dated, nor "surfer-speak".

It's right here:
Siehe Wörterbuch: what a bummer

and means "wie schade".

I think it would fit in this context.

"It's such a bummer that you have to go/leave." I don't think that's slang.
Or, maybe the Leo entry needs to be marked so.

"Bummer!" is also listed on Leo, and not marked as slang.
Siehe Wörterbuch: schade

I definitely agree with eric. The original post sounds as if a colleague is leaving. I would be extremely surprised if someone said What a bummer... in those circumstances. I would certainly class it as slang.

bummer noun 1 slang a difficult or unpleasant thing • She failed again? What a bummer! http://www.chambersharrap.co.uk/chambers/feat...

Bummer, a slang term used by stoners and surfers to express disappointment in the outcome of a given situation http://books.google.co.uk/books?id=0Vf4eyhg5T...

BUMMER. The term "bummer" emerged as a slang expression in the 1960s to refer to a bad experience of any kind. Upon receiving bad news, a "hip" individual would simply utter "Bummer," ... http://books.google.co.uk/books?id=jUfiMkBSMr...

bummer noun 1. Slang A great disappointment or regrettable fact http://books.google.co.uk/books?id=dF2WpFK3A5...

Looks like it might be worth adding some tags on Leo, though the first translation "So ein Mist" gets the idea across pretty well, doesn't it?

Bummer should not be used in a business situation. I agree it should be marked as slang.

To the OP I'd suggest:

I was sorry to hear you'll be leaving us. I wish you all the best in .... (your new job, future)

Bummer is generally considered slang, or at least very casual. It's sometime used in a not-quite-serious, or lightly ironic sense.

See http://dictionary.reference.com/browse/bummer, particularly #2. Also http://onelook.com/?w=bummer&ls=a

It has a very different connotation than the simple meaning of "what a shame" or "what a pity" or "too bad".