So we have sort of accidentally used Foxtons twice now, having sworn that we would never touch them.
When we bought our first house, I found the perfect property on right move but was hugely put off when I saw it was marketed by Foxtons. DH rightly told me to get over myself. We viewed, loved, and ultimately bought the house, and I'm confident that we didn't pay over the odds for it. The whole experience was fine as a buyer.
When we were leaving London we got half a dozen agents to value. Most came in £25k either side of £450k. The awful Foxtons one gave me a stupid spiel, valued it at £500k, and then sat back to watch my reaction. Needless to say we didn't use them. We signed with another London agency who were, frankly, poor; it is worth saying that Foxtons do not have the monopoly on being a bit shit.
We couldn't sell the house in time to relocate as we had to be in our new city for jobs within a month or two, so we let out the house with a local, family-run small lettings and management agency. All tickety-boo for a couple of years, but the owners then decided to retire and sold the lettings side of their company to Foxtons. Foxtons honoured the favourable management rate that we had been paying and their admin was all pretty professional from our point of view as landlords. They were keen to value our property and encourage us to consider a rent rise but we refused. We later realised that they do make a lot of money from selling hugely inflated add-on services to landlords - smoke alarm installation, cleaning services, inventories etc - at the start and end of tenancies.
Our tenants then gave notice a few months later and we decided to sell. At this point, we were living two hundred miles away and it wasn’t practical to get back to London to field loads of valuations - so we went with Foxtons. The higher rate was worth it in our particular situation as the lettings and sales teams were pretty good at communicating and they took care of most things under my (admittedly assertive) remote direction. It is worth noting, however, that they were quite pushy with their high valuation of £500k+. We were adamant that we wanted to market at £475k, which turned out to be about right.
So they were fine for our particular situation, but for OP who is presumably selling her own home I would use somebody local. Do get local, personal recommendations if you can, but specifically from people who have used them to sell. Our disastrous agency were recommended by a friend who had bought with them and it was a mistake to assume that this would translate to a good experience as sellers.