It would be nice to be the lone voice of endorsement in a crowd of criticism but, sadly for Halfords Mile End, my experience was hugely dissapointing too.
I bought a bike there through my bike-to-work scheme. I spent over 500 pounds but was poorly treated, the bicycle was not professionally put together and the customer service was almost non-existant. I had some extra parts fitted with my order such as pin heads to deter theft of wheels and the seat, and discovered that the original quick release parts had been casually disgarded, some of them could not be located. My new bike lock had no key (only discovered when I got home), the staff were unable to fit any mudguards, this dispite persuading me to upgrade from the ones I wanted to more expensive ones because they would "fit the bike better". What really gualled me was that I placed an order, gave the shop two days to put the bike together and still had to wait two hours in store for them to finish my bike.
The staff are unhelpful and try to avoid your gaze. The bike mechanics are underskilled, they missed some obvious things on my new bike, and the bike service desk is a disordered mess. The only person in the entire store who seems to be on the ball in the security guard on the door. Promote him to store manager I say.
One final criticism, no-one seems to have any idea what is in stock and what is gone. Trying to get some extra parts for my new bike was reminiscent of the Monty Python cheese-shop sketch.
In short, avoid.