Q: Computers are convenient. と Computers are convenience. はどう違いますか?
A: The first sentence is correct as written. Convenient is an adjective that describes the noun. You should correct the second sentence so it reads, "Computers are a convenience." I'd say the first sentence sounds more natural.
Q: Computers save us time と computers save we time はどう違いますか?
A: "We" is the subject or active thing in the sentence. It is usually at the beginning.

"Us" is the object, or the target of the action. It is usually at the end.

"We like those people"

"Those people like us"

In your sentence, "computers" are the subject, and there are two targets, time and us.


Q: Computers are essential part of out life now.
Internet helps us to stay in touch with the whole world.
It's more convenient to send email then send letters by post or pay bills and book tickets online while staying at home. But unfortunately some people use computers beyond measure. この表現は自然ですか?
A: Yes! It sounds really good!
Q: * Computers and other electronic equipment become useless in just a few years, leaving customers with little choice but to buy newer ones to keep up.

What is the subject of 'leaving' ?
I think it is "Computers and other electronic equipment".
Innit ?
A: @THE-FIRST The subject of leaving isn't entirely stated in the beginning but it is implied. so I dont think that's completely wrong. the way the sentence is set up, it's showing indications of cause and effect.
Q: Computers have been greatly improved recently. この表現は自然ですか?
A: I would write "Recently, computers have been greatly improved".
Q: Computers have an impact on people's communication various across ages. この表現は自然ですか?
A: @3carina: "Computers have an impact on people's communication across various ages" This would make more sense.