Ooooh, discussion! Well, this is an interesting topic, so let's see....
Are you a good thinker?
Quite so. Extremely so. But maybe not in the way one would define as "good thinking". I don't think about stuffs better than most, I just do it more... controlled, maybe? I can decide what I am thinking about at any given time, which isn't as easy at it sounds, because at times it's really hard not to think about pink elephant. Also, my thinking, even about stuffs that directly concerns me, can be pretty detached. Like I am just random observer to the situation. It has its benefits.
Is thinking fast necessary?
Yes. Though, I guess, that would depend on how fast are we talking about here. For me thinking fast is like split-second decision, like which way to run when you are running away from a horse of zombies. Decisions like that are extremely important, but you don't really have time to think about those. However, with experience, thinking like that becomes instinctive - for example, you would pick escape routes based by some tell-tale signs. And will escape zombies much more efficiently.
And fuck zombies, this is as simple as catching a ball. The act of catching a ball actually involves some high level geometry and reaction - you have to see the trajectory and decide on the best move to catch the ball while compensating for its force. It's a lot of thinking, even for modern computer. We do it instinctively, but that doesn't mean we don't think about it - it means we are trained to think fast about stuff like that.
What about thinking slow but rationally?
That would be more classical definition of thinking. Just sit, like Auguste Rodin's Thinker, and ponder stuffs. This is very important, because that's how you really think stuffs through. You have time, so you take in account all the possibilities, all the outcomes... Well, in the perfect case, anyways. In truth, it's not enough to just have time and think hard. You have to think smart, too. If you are thinking about some difficult situation, or some extremely complicated math problem, you just may not be smart enough or have enough knowledge to think up something good. Sad, but true.
Another trap here - and this is very common - is when you think of solution to something. And you come up with something... and you think that it's a good solution... And that's it, you are now stuck. You should think of other solutions maybe, maybe some would be better, but you are now stuck into the way of thinking that looked optimal for you, and it's hard to break free from that. I think, this is viable when imagining some designs, or writing a story. You may think that plot twist you made is good, and it's hard to think about changing it. So yeah, thinking hard and long can lead you into a dead-end if you aren't flexible.
What is the pros and cons of this?
Hard to answer, really. As you can see from me descriptions, for me those are really two very different things. Thinking fast is more split-second instinctive deciding, and thinking rationally is serious pondering. Those two don't mix.
I'd say, pros of both situations are obvious, but cons...
For thinking fast con is that you don't have any real certainty that you made right decision until you see the result.
And for thinking slow you may just overthink things much more than it's needed, which is also bad.
So yeah, I guess, it all depends on how much time you have, and how serious the situation is. Ideally, you want to have just right amount of time to think, because having more may make you start to overthink, and it all goes to hell from there. At least that's my opinion - thinking too much can be bad.
Especially if you take too much time thinking where to run from zombies. They will fucking get you.