This is from a few years ago. We were looking for a quick sale of a property that needed quite a bit of work doing to it (including replastering, new kitchen & bathroom, new windows etc)
We approached one of these companies and also looked at selling it by auction.
Quite coincidentally, we heard back from the company while the valuer from the auctioneers was actually looking round the house.
The auctioneer advised us to go with a low reserve to encourage people to take an interest and that bidding would then bring the price up to a more realistic level.
The offer we got from the "we buy any house" company was way below even the low estimate that the auctioneer suggested we place on the house to encourage bidding at the auction.
In the end, the house went for way above the reserve and actually quite close to what similar properties were being sold for locally at that time by doing it normally.
For actual numbers, It sold at auction for £170k (less fees) after having a reserve of £120k. I can't remember the exact offer from the company, but it was around £100k.