I often hear from customers new to cycling that they don't want to spend much money because they aren't sure they will enjoy biking enough to stick with it. They then shop the discount stores to save a few bucks. It's a logical justification on the surface, but I find that it is a self-fulfilling prophecy. If they do buy that department store bike they certainly won't enjoy cycling, and therefore will not stick with it.
Department store bikes do not fit well, are heavy, poorly assembled, use inferior quality frame materials, and inferior quality components. They are uncomfortable to ride, and in many cases, unsafe.
First, examine the sizing. Department store bikes do not come in the size ranges that bike shop quality bikes are available in, and will not fit most people. Correct sizing is critical to enjoying cycling. Most department store brands only offer one frame size in adult bikes. Bike shop brands offer at least four and usually five different frame sizes to fit everyone, regardless of height. Imagine a shoe store that only sells size nines.
The frame is the most important part of a bicycle. It is the frame that will determine how well the bike will ride, how well it absorbs shock on rough roads, it's strength, and how long the bike will remain usable. All steel or aluminum alloys are not created equal. Department store bikes cut corners by using cheaper alloys to make the frames with. Because the tubes are made from a weaker material, the wall thickness must be increased to compensate, and frames with thicker walled tubing will be heavier and will transmit all the road shock through the frame to the rider, creating a rough, harsh ride. Heavy bikes have a dead ride, do not accelerate well, and are much harder to pedal uphill. (Please read the article on bike weight, also in this section). The frames on bike shop quality bicycles are made from superior materials. The better quality alloys, whether steel or aluminum, combined with better manufacturing processes, such as heat-treating, guarantee a smoother and livelier ride, and a lighter bike that will last for years.
The parts on the bike are also important. On the discount store bikes, the cost cutting here continues. It's not just performance that suffers, such as poor shifting or energy sapping drag from poor quality bearing sets, sometimes it's a fundamental safety issue. The brakes on the department store bikes, even when adjusted correctly, require far more stopping distance than on a quality bike store bike. If you really want to live life on the edge, I would suggest skydiving. Your bike should stop when, and where, you want it to stop.
Every bicycle has to be assembled by someone. At the discount stores the bicycles are usually assembled by the same employees that assemble the pressboard bookcases, the swing sets, the barbeque grills, and the patio furniture. These assemblers usually have had no formal training. Often, they are paid per piece, meaning the quicker they can throw one together, the more they get paid. Bicycles aren't toys. They require precision tuning to assure an enjoyable and safe lifetime. Bicycles come from the factory partially assembled, and generally, this partial assembly has been done either by machines, as in the case of wheels, or on an assembly line. At the discount stores, they assume someone else has done their job right, and simply finish the assembly, as rapidly as possible. We find much of this partial assembly to be substandard. For example, many of the bearing adjustments are too tight, which if not corrected would cause premature wear. Spoke tension may not be ideal, causing spokes to fatigue, and weakening the strength of the wheel. This list is long, as each part on the bike requires special attention during the assembly process. Our mechanics work on bikes for a living, and have been trained by more experienced mechanics that take great pride in handing down the knowledge they have acquired over the years. We never assume that an adjustment is correct unless we have checked it for ourselves. We do it right the first time so we don't have to do it later.
Department store bikes offer no after the sale service, (or during the sale service, for that matter) and many bike shops will not work on them due to liability reasons (see related article in this section). There's no one there to provide guidance or offer suggestions. There is no one there to fit the bike to your needs. There is no one there to service your bike when something goes wrong. There is only a clerk at the sales counter waiting to ring you up.
In terms of skill, fitness and expertise, cyclists tend to grow into the level of whatever they buy, and usually exceed it. It is much more common for me to talk to a customer a year after the purchase of a good entry-level bike, and have them tell me they wish they had purchased an even nicer bike, than to hear a lament that they feel they had overspent. Good, entry level bike shop bikes aren't much more than the discount store bikes, come completely assembled by professionals, and are sized to fit you. The bike shop bike will be ridden more frequently, for longer duration, and will last years longer. But of course the most important point is the quality of the time spent on the bike. Have you ever left a movie theater and realized, with remorse, that you wasted not only money, but two hours of your life? Quality time is hard to come by. Get educated. You wouldn't want to waste valuable time, or money, seeing a bad movie, so don't spend it riding a bad bike.