I do wonder how many of the "I wouldn't touch SLI" people on this forum have actually used SLI recently. It's not scary or difficult.
I'm seriously considering upgrading my 970s to a pair of 1070s. In the worst case scenario, I'll get 980Ti like performance, and in the best case, I'll get 150% of the performance of the fastest single chip GPU available. I expect it will be the latter for most of the games I'm going to be playing.
Value wise, I should be able to get a pair of 1070 FEs for 133% of the price of a 1080 FE, but see gains of up to 150% (£800 vs £600).
The 1080Ti when it comes will probably be about the same performance, but it's not here yet, and before that we'll probably get a Titan at ~£1200 which will have similar performance (at 1440p) to a pair of 1070s.
There is no right or wrong answer to the "should I go SLI?" question, it comes down to a number of different points.
1) What resolution and frame rate you are looking for. Does the current single chip do enough for your needs? I'm looking to push average frame rates of 144fps at 1440p without compromising on quality settings.
2) What your current setup is. IMO, there's clearly not much value in upgrading from a 980Ti or 970 SLI to a 1080 for £600, so the next noticeable speed bump is either 1070SLI, 1080Ti or TitanNext
3) If considering SLI, are the cards you are choosing good enough individually to still have enjoyable gaming if a game does not support SLI, or in the unlikely event that one card fails. (If a single card user's card fails they have either nothing or intel IGP to fall back on)
4) What size case and PSU do you have. Clearly SLI is not suitable for SFF.
5) Does the *possibility* of micro stutter worry you. In my case no because of GSYNC
Each of us will have different answers to these questions, and therefore the choices will differ.