I've visited Cycle World, Custom Bicycle Shop, and Reseda Bikes. I haven't bought enough things from the three so all I have are mere impressions.
Cycle World was helpful. I've bought lights, toe straps, a chainbreaker, and spoke wrench from them and the service was good. They will give you or point you to what you ask for. One day I asked about rims and one guy took me in the back and had me browse their new old stock rims that they wanted to get rid off desperately so he offered to sell them for low discounted prices. About 2 months ago, they had a clearance bin but was gone when I came back a few days later.
I've visited Custom Bicycle Shop a couple of times but never bought anything from him. He was busy working on a bike while talking to another customer but asked me immediately if I needed help. I told him I wanted to convert my one piece crank to 3 piece and needed the adaptor and his immediate response was "why?" and "bring your bike in". He's the type to make sure that what you're asking for is what you really need. He has some nice used stuff in the shop and I think he has more in a storage so it's worth giving a shot if you're looking for something used and hard to find.
Reseda Bikes tend to let their customers browse without asking them if they need help. They want you to feel welcomed to just browse and look and figured that a customer will ask. This is not consistent though. There have been a couple of times when they asked me if I needed help. It seems to depend on how busy they are or if you look like you already have an idea of what you're looking for. If you walk in and go straight to the panniers then they probably wont ask you if you need help. I've bought gloves, tubes, tires, patch kit, a chain, toe straps, and rear rack and the service was good. The owner is the type to also make sure that what you asked for is what you need, plus, what he thinks will work better. I was asking about a front rack but suggested I get a rear rack instead.
I will continue to visit all three but will go to the one depending on what kind of service I need that day.