I worked in and managed a bicycle shop for around 13 years. Cheap bikes when I was a kid were $100, and yet these a couple decades later are still only $100ish. While they weren't good back then, they've gotten even worse now. They are built to an extremely low standard, and tend to fall apart even if you're not doing much in the way of riding.
There are two major problems to this type of bike, though.
First, they are never built properly. Part of the reason a bike at a bicycle shop costs $250-300 is because it's built by a trained mechanic. An improper build leads to bearing going bad, poor shifting, stripped threads, and brakes that don't work properly. This also causes everything to wear out that much quicker. That, and the 45+lb. weight of these things is ridiculous.
Second, they only come in one frame size. Any bicycle from a shop comes in at least six sizes to fit a range of people. This is critical to make the bike enjoyable to ride, as a poor fitting bike will either be too cramped or stretched out, and thus, very uncomfortable and inefficient to ride.
These two factors together make for a miserable riding experience, even just around the block. I can't tell you how many people buy these cheap bikes to get back into riding, then end up hating cycling because the bike fits terrible and is always falling apart. It's not until they get on decent bike that they start to enjoy it.
To summarize - avoid these bikes at all costs. If you can't swing a few hundred for a bike shop quality bike (which I totally understand), get a used Trek, Specialized, Giant, or other quality bike off craigslist. This was always my recommendation to customers that came in but thought our bikes were too expensive. I genuinely don't care that people ride the best stuff. I care that people enjoy cycling.
Edit - when it comes to kids bikes, I totally understand buying one of these cheaper bikes that they'll beat up and outgrow in a year. Just PLEASE makes sure you know how to properly assemble it. If you don't, have someone that does check it over or take it to a bike shop.
shabazz18 wrote:What exacty makes these so bad they can be used for nothing other than riding around a neighborhood? Specifically the more expensive Titans. I had nothing but cheap bikes growing up and beat the hell out of them. Only thing i ran into was a popped tire or the occasional bent rim. I have a feeling the people knocking cheap bikes are the people that run Fuji's or other astronomically priced bikes.
Keep in mind the people in the market for this sort of bike are not going to be running them down double blacks on a regular basis.