Evaluating tactical situations in Puerto Rico
Captain: I would argue that this is actually the most complicated role in the game. If calculating its effects seems easy to you, then Captain might be getting chosen too often in your games. In the group that I play with, it is usually not cut and dry...ie...not all goods will be shipped.

When that happens, so many things come into play:


Which of my opponents is likely to
captain if I don't?
Will I still be able to ship all my
goods if they captain instead?
Can I force someone else into loading
a coffee or tobacco boat when they
would rather not?
Will the other players in the game
understand and silently collude to
make that happen?
Can I start a coffee or tobacco boat mid/late game to block my indigo and corn shipping opponents?
Who has harbors, and how many times
will they get to ship?
How full is each boat and will the
other players get to ship what they
want?
What about warehouses?
How will the leftovers affect the
next trader phase?
If I delay captaining now, will the
next craftsman cause a shortage of
goods (ie. the supply will run out)?
Will the players with factories be
affected by the shortage (and get
shortchanged on doubloons)?


Trader:


If trader is a great move for me now,
then it's likely that other players
won't want to take it. I can wait a
turn and it will have accumulated an
extra doubloon.
If I don't trade, will I still get to
if someone else trades (often the
answer is yes).
What goods are players left with
after a trade and who will be set up
well for the next trade?


Mayor: the other responses in this thread make good suggestions about mayor.


Look for a big gap in benefit
(getting 3 colonists vs. 1
colonists).
Do other players want me to mayor? If
I've got early production (corn and
indigo), and they are sitting on
coffee and tobacco with unmanned
production facilities - there is
almost no way I would ever choose
mayor. Force them to choose mayor, so
it will delay their craftsman by a
whole rotation.


Craftsman - Many of the considerations are actually covered in the Captain section. But also:


The benefit of taking craftsman is
only one extra good, which is not
that significant most of the time.
When you take it, you are setting up
all the other players to have huge
moves on the subsequent turns, while
you are in the worst position.
If you don't craft, will you get shut
out because of good shortages?
Do you have a factory? Do not craft
just so you can get your extra
doubloons. Someone else will craft,
and you will still get them.


Settler: I try to always take it in the first two or three rounds if I can get a quarry. After that, it feels like the turn you give up to take it is almost always a waste. You'll usually be able to get what you want anyway when someone else chooses settler. If your strategy depends on getting a specific mix of plantations, then you might want to reconsider how you play the game. Which plantations come out is the only element of chance in Puerto Rico, why put yourself at the mercy of it?

Builder: Most of the time I feel like this is one of the easier roles. I usually only take it if:


I can build and my opponents cannot.
Passing will give an opponent the
chance to buy the last of a
particular building that is crucial
to my own strategy.
The 1 doubloon discount will enable
me to buy a key building one or two
rounds earlier than if I let someone
else take it. (although as mentioned by Lance, you might be able to get this doubloon some other way on this turn - often by taking another role with a doubloon bonus already on it)
It is mid/late game and I am trying
to end the game by filling up all of
my building spaces


.