Using a Great Merchant to research a tech is a waste, IMHO. The other two uses are awesome:
(1) Trade mission, which is better in the late game and can net a massive amount of gold. I've gotten over 6000 before... the amount you receive depends on the distance from your capital and the size of the city. I'm guessing the game speed affects it too, but you can't really control that in the middle of the game. Upgrade your troops, run at 100% research for a while... if you've got Universal Suffrage, this money can be translated into hammers in any city (even better with Kremlin).
(2) Super specialist, which has quite a variety of applications. If you want to take advantage of the gold he generates, drop him in your Wall Street city and get up to +18 gpt. Alternatively, you can use the food he generates in your GPP farm to help support even more specialists, which could led to even more GMs. The ultimate combo would be a holy city with shrine, Wall Street, National Epic, and a high population. Drop a few merchants in there and you've got a powerhouse of a city.
I just thought of a nice use for the trade mission in the early game... If you have the Pyramids and can access Universal Suffrage, you can probably hurry a sizeable medieval/renaissance army with the cash and dominate a few opponents. Might have to try that sometime...