We spoke to them last year and went through the process up until signing up.
At first I was sceptical as it did seem too good to be true but when we went through the figures we could see that it wasn't (although it was still competitive)!
From what I can remember, the agency fees are 2-3% (we were paying 1% on the high street) and obviously you also only get 95-97% guaranteed which, when we are talking about house prices, even 1% is a lot of money to lose out on. If they sell at or above your asking price (ie 100%) then you get 80% of this.
The other thing that didn't work for us was the 90 day period. Our house move problems swung between having a buyer and not having a place to move to (so if we used Nested and not have anywhere to move to after 90 days we'd have a problem as they only advance the money for a purchase) vs needing to move quickly but no buyer (so in that case, if it's within the 90 days you have to pay extra to get the money earlier).
Another thing to consider is they only take on houses within the M25.
I do know a regular suggestion on MN when people talk about "we buy any house" (which Nested are most certainly not like!) is that you should lower your house price and you'd sell quicker and not take as much as a hit as going through them. With this there is still no guarantee that that offer will proceed whereas Nested guarantee you the money after 90 days.
What I would say is that I was massively pleased impressed with everyone I spoke to there. Not only do they know their stuff but it really felt like they wanted to help (ie they offered to review the property we were trying to buy to ensure we were offering at the right point etc even though at that stage we had told them we wouldn't be putting our house with them).
I think it's a fantastic concept and will really work for some people who need the certainty and are willing to take a bit of a hit on their sale price (to pay their fees).