I've had bad experiences with International Bicycle Center in Allston (on Brighton Ave.), mostly because they're loathe to repair anything on my Raleigh. But even for basic work, I've had them give me back the bike with the fenders on wrong, and other minor problems like that. That said, I do go to them to purchase helmets, locks, lights, etc.

The best bike shop experience I've had in Boston was at Broadway Bicycle School, a cooperatively owned shop and repair school in Cambridge, near Central Square. They tore apart and repaired my hub and now the bike rides better than it ever has. And they did it quickly and cheaply. And it's a coop, so you know that your hard-earned dollars are going to the guy who fixed your bike, not disappearing into some corporate ether.