Buying wisely from Halfords can get you a decent bike. The Carrera range are great value for money - so much so that the local oiks have nicked three from our garage.
I've had two Carrera Krakens and my son is on his second Gryphon. I only migrated to a Trek MTB because I didn't like the redesign of the Kraken.
I've assembled three out of the four bikes and I had no problems with the one that was assembled by Halfords.
There are variations in the levels of competence of the bike staff - some i suspect are filling in from the car division judging by their indifference to all things cycling related but to be fair I've been happy with the staff in the two Leeds stores that I have used.