I just had a look at the Eurostar website. I put in a journey for standard class London to Amesterdam and as said by others, it requires a stop in Brussels (that's the end of the line for Eurostar). It's about 3 hrs 20 to Brussels and another 2 1/2 hours to Amsterdam with a 25-50 minute wait in between. So you're looking at about 6 hours of traveling at a cost of about £174 for 2.
A similar trip to Brussels costs £138 for 2.
I guess it depends on how much of a train enthusiast you are, how much time you have, whether you're afraid or don't like flying or are environmentally concerned (are Eurostar trains electric?), etc.
A flight could be cheaper and faster (depending on how easy it is to get to the airport and where the airport is on the other end). Then again, train stations are often in the center of cities.
Matt