It's a nice idea, the Eurostar, but I honestly can't see the point
. It's not that much cheaper than flying, although a lot longer (3 hours from London
, according to my esteemed colleagues).
Unless one's circumstances are extremely strained (in which case the bus and the ferry are much more viable options) it really is a bit of a gimmick for tourists.
On the mainland itself, now, the TGV (tres grande vitesse, or "very fast" trains) is a boon - one can get between destinations which are not served by domestic flights in about 2/3 the time it would take on a normal train, and about 1/2 the time it would take by car. Very handy.