What are the pros and cons of SLI?

The only pro is that you have the chance to double your graphics performance.

Cons: A lot of games dont work with it or do not work well with it. A lot more heat. Power consumption. Noise.

Generally the cons out weigh the pros, especially if you can afford a better single card solution.

Requirements:-
1). Power supply capable of powering all the components in your system, if you bought the original power supply based on a single card it may need upgraded.
2). SLi capable motherboard with enough spacing between the slots to mount your graphics cards, the slots themselves must be capable of running in an x8/x8 configuration so the data flow is balanced between the 2 cards. A x12/x4 setup is no good as it hampers the second card.
3). An SLi bridge.
4). Enough airflow through the system to cool the cards.

Pros:-
1). Almost double the performance of a single card.
2). Cost effective to increase performance at a later date.
3). Provides addition sockets for a multi-monitor setup.

Cons:-
1). Higher power requirements.
2). Produces more heat.
3). Not all games support it, you may get performance equal to or worse than a single card in these cases.
4). Sometimes requires a larger case for the extra card.
5). Some setups exacerbate micro-stuttering which can make a game anything between distracting and unplayable.

Personal recommendation:- if you can afford it go for the single most powerful card you can afford. I have a GTX 980Ti, this is roughly the equivalent of a pair of GTX970s in SLi, without any of the drawbacks or requirements of SLi.

The only thing close to a definitive answer you can get is this:

-If you want high performance and don't mind tinkering, multi-GPU all day every day.
-If you want simplicity and don't like messing around with your system, you don't want multi-GPU.

SLI and CFX are only worth it if you don't care about spending the extra money on hardware & electricity, having extra heat output, occasionally dealing with incompatibilities (that you may or may not be able to address using third party utilities), and have a base system capable of handling the setup in the first place (this means sufficient PSU, CPU, motherboard & cooling).

Basically, what camp do you fall under: Enthusiast or Casual?

As someone who actually has ran both Crossfire and SLI for many years, let me put an end to most of the baloney I see posted when people ask about SLI. Here are my basic facts, in my opinion.

If you have a low end card now, buy a new best single card you can afford. Run it until you feel the need for more speed, then buy another and put it in SLI with the one you have!
If you have a mid to high end card now, and have a PSU big enough to run it, get another card for SLI.
I have never experienced any stuttering or other nonsense that I see people posting about that I have been able to notice, let alone make it "unplayable".
I have never seen a game that didn"t support SLI. Most all modern games, of any popularity at all, support SLI.
However, each game will net you different results, some games respond very well, others not so much. Games that are optimized well for SLI will have frame rates 20-30% higher than a 980ti, games that are optimized poorly will play about the same, or about 10% poorer than a single 980ti.
Its up to you to decide if what you play, and the money outlay for another 970, or something like a 980ti fits your need better. There are numerous benchmarks comparing these setups around the web. I just want to say that from my personal experience, that "cons" I see are for the most part, lean towards the severe, worst case problems. SLI runs hotter? Yes you have 2 cards in there, but is that really a problem? Not really. Micro stuttering? Yes, it is a fact some cards in some setups playing certain games can experience this, but there is a reason its called "Micro", its really hard to notice, if you even have it at all. I have never had it in any of my setups, that I ever noticed.

In my opinion, if you already have a 970, and your PSU can run it, another 970 will give you a lot of bang for the buck over buying a single card that will will beat 2 970s in SLI.

I have three questions:
1. What resolution are you going to play on?
2. What motherboard do you have?
3. What power supply do you have?

The rest of the details about the pros and cons have been listed.

1. Scaling (fps) will increase up to 96%.

2. Some games will not scale well, this is generally because vendor had no interest in creating a SLI profile ... usually because it wasn't a very demanding game and was getting way over 60 fps w/ a single card. You don't get SLI to take a game from 90 to 150 fps ... you get it for the times when you have 30 fps.

3. Performance requires power and power produces heat. Twin 970s in SLI outperform and produce 8% more heat than a 980 Ti.

4. The performance / cost ratio is always higher than a faster card. Here's the ratios (fps per dollar) according to techpowerup's data

970 - 3.97
970 SLI - 3.48
980 - 2.97
980 SLI - 2.60
980 Ti - 2.53
980 Ti SLI - 2.21

5. The amount of noise produced is comparable to the heat produced... again SLI'd 970s produces 8% more heat than twin 970s. About 95% of the builds we are involved in are either SLI from the getgo, or designed (MoBo and PSU) to go SLI at a later date. My son built an SLI 970 rig, his college roomate has a 980 Ti ... roomie bought another Ti within a week. The only thing you can hear in my son's build is the PSU power supply fan (EVGA G2 1000).

6. Been building PCs for 25 years, never seen microstuttering on any build we've done. It was possible years back when nVidia enabled SLI on budget level cards but this hasn't occurred since before the 4xx series. Yet, forum posts today from peeps who never used it continue to parrot these and other issues from past eras as being relevant today, they are not... just read any current SLI review. The worse thing that can be said is that a game might never support SLI (usually because it isn't needed) or that it might not at game launch.

http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_970_SLI/22.html

7. Requirements would include a decent 750 watt PSU and a SLI caable MoBo which will come with the necessary bridge.

8. I recommend one 1200 rpm case fan for every 150 watts of power (5 in this case). Less of course if you use higher rpm.

9. I'm typing from an SLI rig (meter says I'm pulling 118 watts) we have 3 SLI rigs here, we've built many, many more SLI rigs for others. As yet haven't had to do any "tinkering" or "messing around".

10. Given the cost / performance ratio, the only reason I would ever recommend a 980 Ti is if, along with it, you bought an SLI capable MoBo and PSU so that, down the road, you could buy a 2nd GFX card and thereby extend system life 18-24 months.

I will leave you in the hands of these two experts. In all the years of building, upgrading and tinkering with my own rigs I have yet to get the flawless SLi setup these gents claim to have.

Out of all your responses... This one seems to be the best... I've done Crossfire for many years... The only issue I've ever faced was the increase in noise level from having two cards instead of one... but yes, an SLi 970 would kick butt.