I looked at a few and had a decent conversation with one when we were selling my dads house. There 7 of us involved and I assumed things could get messy. The house was valued at around £560k, it needed work..... and we were offered, deal done completely, in 30 days, no expenses, no hold ups, guarranteed sale, all legals sorted, no last minute deals and no last minute change of mind... £420k and £440k so about 80% of market value. They have lists of builders and investors and presumably they sell the house on to these parties. I was sort of outvoted but we did sell quicker than first thought through a reputable estate agent at the market price.
I then considered using them again for the sale of our holiday rental property, taking into account we could better invest the money elsewhere. Taking into account a 6 month contract with our rental agent and loss of earnings on potential interest, any remedial decoration, loss of interest when money invested over a given period of time and any other costs involved there wasn't a lot of difference between what an esate agent would get and a house buyer would give, we arranged an amicable deal with the agent so we didn't lose income whilst it was on the market.....I suppose it all depends on how quickly you wish to sell and what your situation is together with the condition of the house.