I think you'll get varying opinions on the Paris for a day so you just have to weigh if it's something you want to do
I have been to Paris several times (I grew up in Europe) however my DS and DH have never been. We are staying 4 nights after our cruise (TWO WEEKS TO GO!) in London after the cruise, however one day yes we are taking the Eurostar to Paris. We are taking the first one at 6am and the last one back from Paris at around 815pm. I hired a private tour guide for 7 hours after we see the Eiffel Tower on our own and he will then make use of our limited time and will have us seeing a ton of things that I know are awesome.
I'm not sure when they will get the opportunity to be so close and a little over 2 hours each way is awesome to me and also the Eurostar is part of the experience.
We are excited about it so I'm in the "go for it camp".
Heather