I'm not quite clear on one aspect of this question: positive and negative effects FOR WHOM? Are you asking about the effects on the people who rule the empire or on the ruled? I'll answer this with a focus on the ruled.
For people who fall under an empire, there are positives and negatives. The most usual potential positives are 1) better economic opportunities and 2) more security.
Empires allow for better economic opportunities (especially in the old days) because they allowed for trade between various areas. Trade between countries could be banned or be too dangerous, but trade within an empire would not be.
Empires can provide better security for many people because they will not be subject to wars going on between countries. This was the idea behind the "Pax Romana" during the Roman Empire. Instead of having lots of countries fighting (and potentially destroying your crops and property) you get peace.
The negatives are that you get ruled and the rulers will care more about what's good for them than what's good for you.