For us, at this point, it's another yes. Here's why:
1) The mugs are so happy! I use them every day at home. So even if you don't drink enough to get the full value at WDW, it's still a fun reminder of your trip and could be useful at home (especially if you don't already have lots of them in your kitchen already!)
2) My husband and I drink coffee in the morning, and I like to carry the coffee with me to the parks.
3) We don't usually return to the resort in the middle of the day, but we're probably going to do that in July, so we'll get sodas or lemonade mid-day and by the pool.
4) We have coffee or soda in the evening while our boys swim.
5) Even though you can't get free refills in the parks, the mugs are useful to us there. The sodas we buy in the parks are HUGE. So we wash out the mugs and split the sodas.
6) I like to carry something that is relatively sturdy and doesn't spill, especially chasing after the kids!
But the main reason we can do all this is because our youngest (pre-schooler) is still in a stroller, so we throw the mugs in the stroller basket. When he's too old for a stroller, I'm not sure we'll still get the mugs.
So far, we've only bought them for my husband and me, but not our older boys (pre-teens.) We're considering getting mugs for them on the next trip, so that they can have something to carry around for water and then get drinks by the pool.
Good luck deciding!