It depends on which great person. Great Engineers should pretty much always be used to rush a wonder, Great Scientists are typically used to bulb a tech, and Great Generals are used for their combat boost with your army during war.
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I find that early Great Scientists are often better used to build the Academy tile improvement. +6

doesn't sound like much, but it is modified by all the buildings etc. In the early game, bulbing a tech to save time may seem like a good idea, but in the long run it may be a waste. I have had games as Babylon where I settled all of the GS up to the Renaissance Era and had 10 Academies outside my science city. When using this technique, make sure to have a mountain next to the city (to build an Observatory) and plan to put the National College in the city as well. By the late game, the Academies will be pumping out 8

each modified by the University (+33% or +50% with Rationalism/Free Thought policy), Observatory (+50%), Factory (+25% with Order/Planned Economy policy) and National College (+50%) buildings. This can be further modified by adoption Rationalism/Sovereignty (+15% total

) to produce about 25

every turn! Over a long game, that adds up quickly and can often outpace the cost of shaving a few turns of an early tech considerably.
There are times when bulbing for earlier tech may be better even if the

generated is lower. I often bulb to enter a new era (to open up new policies), to make a building available earlier (to get benefits from using the new building faster), or to get a tech for a wonder that I want to build.
Using a combination of scientist specialists, high population (with Library and Public School), and several academies, I have had a city produce over 500

per turn.