@tegofc
It's one of the best starting locations I've ever seen. Seriously.
There are several rules of the thumb when it comes to great people:
1. Scientists
Build academies around the city with NC, which is usually your capital. If you have cities with an observatory, they are as good. Since industrial era, it's better to bulb them, i.e. use to discover an advancement early. Remember that you can queue your research on the tech tree with shift-click. This way you won't waste excessive hammers on something you don't want/need tight now.
2. Engineers
They are best used to hurry production in a city. You get (X + Y*citizen) hammers when you do so which is important because bigger cities will allow you to build bigger wonders. If there are no useful wonders available and there won't be any for quite some time, improve a tile because it's always some additional production and it also eliminates unkeep for the unit.
3. Artist, writers and musicians
Being able to defend yourself against enemy influence is important in BNW so all early units of these types pretty much must be used for their great works. Try to time their appearance with appropriate buildings. In late game, their instant benefits may come handier, though if you are going for cultural victory, remember that their tiny bonus to tourism will be multiplied by many factors so it's not the basic +2
4. Merchants
Aka the useless ones. Their tile improvement is negligible and their trade mission is only slightly better. Try not to earn any.
5. Generals and admirals
Admirals are useful merely as combat aid on the sea. They help, though if you don't have +2 movement for them, they can be a burden. Generals are much better because they can steal land. sometimes it's the most important feature for me, in other cases I want to block certain route with a citadel, and sometimes they just stick with armies. They are their own specific case.
6. Location of tile improvements
It's usually the best option to place great people's improvements on strategic resources because they can connect them and will also provide small tile improvement bonus. Note that it does not work with luxury resources. In early game, make sure that you are getting enough food so if you have an empty grassland and a hill with iron to choose from, the grassland may be a better option.
7. Tile improvement vs. an immediate benefit
There's one more thing to remember besides what was already said. On one hand, if you are planning to go down the freedom path, you'll be doubling the quality of great people's improvements, and if you will also be able to create numerous great people in your empire, the combined benefit will be huge. On the other hand, if you won't be getting any additional bonuses from such tile improvements, it's obvious that you should not focus on them too much.