Halfords - personal experience...well. I bought a Raleigh Tundra from them a few years back, my partner bought the same bike at the same time. I was just getting back into cycling and admittedly didn't have a clue, however, the bike went back because the gears were slipping badly. I didn't know how to adjust them, so thought they could sort it. It came back out, the gears were still slipping, but not as bad so I ignored it till the 6 week service...The bike came out with the worse than before. At this point, I had pinpointed it to the rear derailleur, which I did point out to them, the front was fine. The mechanic was adamant it was the front, his way of determining this was to just keep running the chain at high speed over all the back cogs and wait for it to jump off (this was actually witnessed by someone from the old C+ forum, as I found out afterwards). He then attacked the front derailleur with a pair of pliers and bent it outwards as far as it would go and said this would fix the problem, the rubbing brakes were adjusted by hitting the pivots on one side with a hammer. I was getting seriously concerned at this point and decided to just take the bike away before it got subjected to more torture. The gears now slipped at the front as well, but worse than that, the bike started to make a very loud groaning noise from the front end that vibrated all the way through the frame...after trying to lube everything I could think of, I realised the front reflector had gone missing, and where it had been, the seal round the headset (this was a quill stem, so not quite sure what exactly it was, but some round rubber bit round there) had disappeared as well. I took it back to Halfrauds, pointed out the problem and was advised it "must have blown away in the wind". He then took a screwdriver to a nearby appolo bike, used it to poke out said bit of rubber and stuck it on my bike...it was about 2mm wider in diameter, so just flapped loosely round the stem, not fulfiling any function whatsoever...At this point, I thought I'd just take it to the lbs as clearly I wasn't getting very far. They happily advised said piece of rubber was an integral part of the headset and couldn't be replaced unless I bought a completely new one, also the front derailler was shot after the pliers treatment and various other bits I can't remember were wrong with the bike -to put it right would have basically been almost the cost of a new one (we are talking a cheap bike here). So back to Halfords, and after a lot of arguing, requesting the manager, threatening with trading standard, etc. they actually offered an exchange. So that was me sorted - new bike went straight to lbs to be checked over, and apart from them not having lubed anything on it and forgotten to tighten a few bolts, it was fine.
After all the problems I'd had, my partner decided to skip the free service and get hers done at the lbs, it was groaning badly at this point as well - closer inspection by lbs revealed it had been built with most of the bearings missing. Admittedly, a manufacturing fault, but one that halfords should have picked up on. She decided she couldn't face the shop again and just footed the bill at the lbs.
Other people's experiences - they all seem to run along the same lines, although it appears to depend very much on location. Friend bought a carrera subway there last year and they didn't have the women's 16 in, which is what she needed. However, the guy told her it was a proper girls bike with a step through frame, which she really didn't want (and which is blatantly untrue), and convinced her that the men's 18, which they did have in would be fine. She bought it - the setup is odd to put it mildly, the bike is big enough for me to ride comfortably and it gives her backache, but hey... They sold another friend a 700cc tube for a 26inch bike and said it would sort of fold up inside the tyre and be fine...
As for the racing team, I know they are quite good (or were last year, no idea what they are up to now), but to be quite honest, I find it hard to develop any enthusiasm for them - the sight of the Halfords logo is enough to put me off. Having said that, they are good if you need a tube or something on a Sunday afternoon when everything else is shut....
Sorry about the lengthy moan - having said that, I would consider buying a bike from them again, as long as it's a boardman, gt, carrera, etc and as long as they aren't the ones building it!