I've been doing this for some time (but been getting slack of late at the office).  Time to get back into it!

My partner and I often appear at our local (near our home) with mugs in hand - the cafe owner is disappointed when we're not bringing our own :)

I've found some cafes don't like filling the mug, even though the cost of additional milk is minimal (and less than the cost of the cup to them!) - but I do much prefer coffee out of a real mug instead of cardboard - tastes so much better!