If you maintain optimum efficiency, then you will not move the second worker into the same tile, you'll have him build a road somewhere else. However, the extra lost worker turn may well be worth it given the circumstances.
You're right. I thought about what I was trying to say later that night and realized that I had left out part of what I was trying to say.
In Civ, you can maximize efficiency or you can maximize speed. In the best of all possible worlds, you'll be doing both. Fast Workers are more efficient than regular workers on 2-move tiles like hills and forests and that adds to their effectiveness and speed.
In a regular speed game, you can either move 2 workers onto a forest tile, losing the turn for both and then build a road in one turn on the next turn.
Total turns for road completion: 2.
Total worker-turns used: 4.
...or you can send a single worker who will lose the rest of his turn and spend the next two turns building the road.
Total turns for road completion: 3.
Total worker-turns used: 3.
The fast worker, on the other hand, can move onto that hill or forest and then build the road on the same turn. That lets you use 2 workers without sacrificing efficiency. Move two workers onto the tile and build the road.
Total turns for road completion: 1.
Total worker-turns used: 2.
That's either a 100% increase in efficiency and double the speed or a 50% increase in efficiency and three times the speed. The big benefit here is being able use a gang of workers effectively. This is especially important when you are hooking up hill and forest based resources with a fair number of workers available (this is very common in the mid game when you build a new city or take over an AI's border city).
The Indian Civ can hook up any hill or forest resource in 1 turn with enough workers (not forested hills, though). Just pile workers on until the job is done. Other civs have to lose turns to the movement while they wait for a couple of workers to build a road to that square so that the remaining workers can move in and do the job. (Note that it does not take any additional game turns for the total job to have the first 2 workers build a road if you have enough workers since all of those other workers would have lost their movement entering that tile anyway).
Clarification: In forested areas (and nonforested hill areas), Fast Workers will build roads (roughly) 1 turn faster than normal workers. It would be slightly more due to loss of efficiency in times of necessity. If on normal speed (2 turns for a road), fast workers will complete the road in 66% of the time normal workers take. If on marathon speed (6 turns for a road), fast workers will complete the road in 86% of the time normal workers take.
All of this assumes that you are going to use 1 worker per tile. Even in quick games with regular workers, it's still often worthwhile to send a couple of workers onto a square to get the improvement done quickly so that the city and the empire as a whole can get the benefit earlier. If I hook up a gold resource in 2 turns with two regular workers instead of 3 turns with 1 regular worker, I get an extra 5 commerce in that city compared to hooking it up a turn later. In addition to that, if any of my cities are past their happy cap, I get the benefit of 1 extra useful population in each of those cities for an extra turn. I'd say that more than outweighs the cost of an extra lost worker-turn due to lost movement. The Fast Worker can get that resource hooked up in 1 turn which means 10 extra commerce and 2 extra useful citizens per

city compared to the way that you suggest.
One of the worst things you can do in the early game is to split your workers. When you are improving resources, if you are mining a grassland gem and farming a corn with 1 worker each, you'll get the gems in 4 turns and the corn in 5 turns. Alternatively, you could put both workers on the gems and get them online in turn 2 and then still get the corn on the 5th turn since you now have 2 workers on the corn. That's an extra 8 commerce and that 8 commerce could be the difference between getting your religion or not in the early game or it could shave a turn off of Bronze Working for you so that you can whip the granary that 1 turn quicker and get the biggest benefits of your corn that much earlier.
I do agree that as a UU they aren't that bad. Although their bonus is pretty small, it lasts throughout the game, so they make up for it through that. All I want to do is clarify to people that their bonus is not as big as most people think (and clarify exactly what their bonus is worth).
I don't disagree with you here (for the most part). I do think that the Fast Worker bonus is quite situational, but those situations do occur in every game that you'll play and if you are piling workers onto your improvements instead of assigning 1 workers per tile, then the Fast Worker bonus is multiplied. Additionally the Fast Workers can zip across your empire 50% faster than their regular cousins and that means you can mine a critical Coal or Aluminum that much faster when it appears in some oddball corner of your empire that has stood neglected. This used to be especially important for Oil once Combustion became available, but now I just throw down a fort on the Oil ahead of time so that I get it instantly when I discover the tech.