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 to Paris, 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.