My wife and I took the trip "round the Horn" a couple of years ago on the Royal Princess.
I posted some thoughts and pictures at www.geocities.com/davidandbella
if you're interested.
We actually found the entire trip to be fairly smooth. Our worst day was travelling from the Falklands to The Horn. That cetainly brought to mind the tales of the roughest water in the World. Many of the passengers were "laid-up" but we used "the Patch" (transderm scop) and were just fine (and I've previously had some bad experiences).
I'm uncertain which route the Celebrity cruises take but they're probably the same as the Royal Princess, in which case you don't do too much in the Stait of Magellan itself, more in the Beagle Channel which is closer to Tierra del Fuego, which is nice sailing (but cold .. see other contributors). Punta Arenas is on the Stait and we were there for a day. It was rough, to the point that the shore tenders were suspended for a couple of hours until the ship could be stabilized with a Tug.
It seems to be hit and miss on this itinerary. We made all the ports but some of them are sometines so rough that the ships just sail on by. Some of the ships on the water at the same time that we were, couldn't make some ports, we were just lucky on the days we arrived.
Good luck. It was my most memorable trip yet.