I wish I had read the reviews online before I used David St. James, because I had a really unpleasant experience with them which unfortunately seems to be the norm with this agency. That is, apart from a few reviews that look too good to be true and misspell most of the words. But you can be the judge of them.
After renting a flat with them in Islington we had to give them a deposit as well as a month's rent in advance. While this didn't seem like the best idea our agent was quite pushy, and as Canadians my room mate and I thought that perhaps was how things worked in the UK. Our move-in date was pushed back several times and on the final move-in date we were told (the day of) that we couldn't move in - the landlord had decided to sell. With absolutely no notice we had a nightmare time finding temporary and permanent housing, and adding to our aggravation was the fact that David St. James made it as difficult as possible to get our deposit back and we had to resort to legal representation. I would not recommend this agency to anyone.