The Eurostar is an interesting way to travel to Paris, and it's fast enough to make a day-trip possible, just a three-hour trip from Waterloo Station to Paris' Gare du Nord. (Note that France is one hour ahead of England, so you will arrive "four" hours later.) We went on to Paris after our stay in London, and oddly enough, we found it was cheaper to buy a same-day return ticket than to buy a one-way. Also, it is cheaper to buy in London than to buy ahead of time in the U.S. We splurged on first class tickets (about $150, compared with about $110 for second class). First class seating is two on one side of the aisle and one on the other. A pair of seats (or a single seat) face another pair, with a table between. This means that one set of seats faces backwards -- be aware of this when you reserve your seats if this is a problem. First class includes drinks (including wine) and a meal. We were served brunch, which was a little better than airline food. You can probably find a better deal on an airline ticket from London to Paris, but the Eurostar departs and arrives in the center of the towns, so you don't need to deal with getting in from the airport or the possibility of lost luggage or delayed/cancelled flights.
The trip from London to the Chunnel is slower than expected -- this is supposed to be the high-speed train -- because the track is not smooth enough for high speeds. The speed picks up as the train approaches the Chunnel. For 20 minutes, the outside is in darkness as the train whizzes under the English Channel. The high speed continues as the train hits the French coast and heads through farmland to Paris.
The train itself is comfortable, and the bathrooms are pretty high-tech -- you hit a button on the outside and the door slides open. Another button inside closes it. A word of warning -- inside, there is a little button with a picture of a key on it. Be sure to hit the button to lock the door, because it does not automatically lock once the door is closed from the inside.