How Good is a $70 Bike?

As a test run, I rode my bike to QFC and back, about 4 miles round trip. I tried to stay off the main roads as much as possible, as I'm still scared of cars. Overall it went pretty well, with some exceptions.

1. The seat was really uncomfortable. It felt like I was sitting on a bumpy, sharp rock. Not fun.
2. The brakes wore down to the point that they were unusable. I almost hit a fence going down a hill because fully squeezing both brake levers did hardly anything.
3. Finally, my 18-speed bike is more like a 6-speed bike. Let me explain. I have a left shifter labeled 1, 2, 3 and a right shifter labeled 1, 2, 3, 4, 5, 6 for a theoretical combination of 18 speeds. In practice, I'm not supposed to use the extreme gear combinations where the chain is not in a straight line. This eliminates 8 combinations: 1-4, 1-5, 1-6, 2-1, 2-6, 3-1, 3-2, 3-3. Then, to make matters worse, I've never been able to successfully shift the left to 3 or the right to 1, eliminating another 4 combinations: 1-1, 3-4, 3-5, 3-6. This leave just 6 combinations that I can use: 1-2, 1-3, 2-2, 2-3, 2-4, 2-5.