I ride a drop bar commuter most days, and I have a more (but not totally) upright MTB as my backup. For trips up to 10 miles, the MTB is fine, but I find that much over that I miss my drop bars and the extra hand positions. But this has more to do with drop (or, I suspect trekking) bars versus flat style bars. The upright thing adds a different dimension.

I've not owned a really 'upright' bike since I was a kid, but I can't quite imagine commuting on one for any distance. I see people using them around the city all the time, but few (none?) of the long haul commuters use them. As others have mentioned, you lose pedaling efficiency, and your weight is all on your rear. It would also seem like a challenge to get up out of the saddle for hill climbing. I guess if it were geared right and you wanted to crawl up hills (or you live where it is truly flat) it might be okay. 5K? No problem. 18K? probably the outer limit of what might be passably comfortable on an upright bike. But ride what works for you.

There are plenty of drop bar bikes with less 'hunched over' geometries, rather than pure road bikes. I'd say consider these...