UKC

With the Siskin, the lightest single rope money can buy, Edelrid are pushing technical limits. This may be a niche product, at a niche price, but its performance benefits could make all the difference when marginal gains count, reckons John McKenna.  

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How does it compare weight-wise to other 8.6mm ropes?  It's cross-sectional area is 97.7% of the Mammut, yet the weight is 94.1% of it.

Edelrid claims this is the world's lightest rope. But it's the same weight as the Beal Opera, according to specs. It would be nice if the review could directly compare the weight of the 60m versions of these two. Don't forget to compare the length of the 60m versions and make sure they are equally long (which I bet they're not). 
That said, I think that the main use case for using this rope is on multipitches where you want to save weight, and where a small diameter is also nice to have when you rap with the tag line, double rope style.

For hard sportclimbing, I think that slightly bigger ropes (Canary, Joker) already offer some extra durability.

My 60m opera's were 63m when new out of the packet. 

I measured them both with a cordage meter.

I thought the Beal Opera triple rated half rope is the lightest single rope? 8.5mm

It's really interesting to compare this rope to the not very successful Edelrid Corbie of a dozen years ago https://www.ukclimbing.com/gear/climbing/ropes/edelrid_corbie_-_for_when_we... The Corbie was also 8.6 mm but 51 gram/m rather than 48 here, so (and I know these figures aren't super-accurate) presumably the Corbie was denser in some way. John doesn't seem to have worn his Siskin excessively, which was the serious issue I had with one of the two Corbies I reviewed.  Looking on the Siskin's product page https://edelrid.com/gb-en/sport/ropes/siskin-eco-dry-8-6mm?variant=4312316 it is actually a triple rated rope, but Edelrid don't seem to be making much of that - call it the "lightest single rope" in the world. So I guess it's just improved technology over the last decade that has let them build a rope that is the same diameter as the Corbie but either has a thicker sheath or at least a more abrasion resistant sheath, rather than them focusing on a thin rope that only was designed for passing the single rope standards, rather than that as well as the half and twin standards (although perhaps if you make a rope that passes the single standard it will by default pass the twin and half ones also?).

The single standard has to be harder to pass, as thinner halves and twins have been available for decades. Rewind to the early 90s - singles were generally 10.5mm, halves were 9mm then.

My first single (Cairngorm Ropes - they didn't last too long as a company. Early/mid 90s) was 11mm!

Still Edelrid related, although not to the Siskin, I was using my Roca Free 10.5 https://www.ukclimbing.com/gear/climbing/ropes/fixe-roca_free_105_mm_rope-7... at Harpur Hill, sport climbing on Sunday. It did not play nicely with my Edelrid Pinch. It's not very furry or beat up, and the Pinch does say it handles up to 10.5, but it was more hassle than it was worth.  I did check - the minimum diameter for the Pinch is 8.5 so unsurprisingly, you should be able to use the Siskin with a Pinch.