I recently bought one of these, which is definitely compact. I haven't actually used it yet, because I don't tend to buy drinks during the normal course of working life, but I'm hoping it will come in handy when I travel and don't necessarily want a mug taking up bag space all the time.

I also have a Bodum stainless steel mug which is really nice except that it does make tea taste a bit metallic, so is less use now I don't really drink coffee any more.