I used Eurostar once prior to HS1 opening, in 1999 to view the total eclipse in Northern France. The route into Waterloo was totally unsuitable, slow and busy. Once HS1 opened in full journey times were slashed. Terminating at St Pancras is much more useful than Waterloo, taking Euston as well which is only 10mins walk, you have every major north of London destination available in one place with no need to use the underground to transfer which saves further time if you are travelling from the Midlands or North.
Since HS1 opened we use Eurostar regularly, if it hadn't been built and we still had to get to Waterloo and then spend an hour ambling through South London on commuter lines before getting onto high speed track towards Paris then we would fly. I cant be alone in this view, which is backed up by the number of people who get off arriving services at Kings X and make for the Eurostar terminal, also the number of people dragging suitcases down Euston Rd from Euston.
As has been aired in other threads Eurostar capacity is limited by facilities at St Pancras to process passengers, I suspect if more trains could run they would be filled, the demand appears to be there.
Finally, anyone who travelled through Kings Cross or St Pancras in the 80s will remember just how bad the surroundings were, compare that with today, the redevelopment that HS1 helped has transformed the area. Just a shame they didnt build the HS2 terminal instead of the British Library, but I realise HS2 wasn't on the agenda when the redevelopment of the area took place.
HS1 also gives better access domestically to Kent
So whilst some of the planned cross channel services never materialised I would suggest that HS1 acheived a lot of positives.
