I went to the Hostel 639 in June 2014 for 4 nights and had a great experience. Breakfast costs less than £2 and is unlimited (grilled bread, butter, jam, spreads, cornflakes, coffee, tea, milk, everything you like!). We booked beds in a mixed dorm for 8 and they ended up giving us a private room for four with its own bathroom. What an amazing surprise!
It's pretty basic and I would not recommend it as hostel for first-timers, but it's perfect for experiences backpackers and very clean (including the sheets and rental towels). We even managed to find different people to go out with at night! There's a tube station right in front of the hostel (Kensal Green, open until midnight ) and a bus stop about 100 meters away.