Went to Paris on Friday, so 2 days after opening. Look like loads were much lower than Eurostar expected, as I could get a £29.50 ticket at 3 days notice for a mid-morning train (and there were quite a few others until mid-afternoon to choose from) whereas the cheapest fares were at £149 when I first looked a couple of days earlier. Check-in was a breeze with virtually no queue 30 mins from departure and so was security. The building is beautiful, especially on a crisp, sunny winter morning but the hype that surrounded the opening spoilt it somewhat by creating excessive expectations.
The overall experience was smooth and pleasant, a good feeling of space (this reminded me of STN before the great LCC invasion), all in all civilised (certainly compared to most airports) but nothing extraordinary. I did not feel like I would feel like spending any unnecessary time in the station, which is fine with me. Functional and 15 mins from home means that it is extremely convenient, and that's great. Not sure how I would feel at a more crowded time, though.