There is a huuuuge difference between an cheap bike and a good bike. The first thing is the weight. A couple pounds does not make a difference, but a 15-20 lbs difference is huge, it allows you to go farther and faster, let's you enjoy biking a lot more. On a lighter bike, you'll look at the hills and say, "let's go!" while on a heavy bike, it's more like, "not another hill..."
The front shock is huge too. On a rigid or a cheap shock, you get bounced around a lot more. You have to pay closer attention to all the rocks & grooves on the ground. I've got a Marzocchi shock on my bike, and rough terrains are far more smoother & comfortable. It's a similar difference between riding in a car with poor suspension versus a car with a new suspension. I can ignore small rocks and sticks on the ground, only focus on the bigger ones, which again lets me go farther and faster with less pain. I've had this Marzocchi fork for about 6 years now, and the thing is still rock solid - it's an air fork and hasn't lost any psi ever, the seals are that good.
And with the better bikes you shift faster which makes going up hills much easier. You brake quicker, which lets you go faster without worry. You also get more points of contact in the rear hub. My rear hub has 36 points of contact, which makes peddling so much smoother and easier compared to the "getting around campus" bike I had that was just 16 points of contact.