I think that you should use as few words as possible while still getting the valuable information across.
I recall one Star Wars game where the PCs went to a refugee planet. In my notes I have written down: "Impress the SIZE and REFUGEES on them." So I probably said something like:
"When you land, everywhere around you, you see the teeming masses of refugees and the crappy little shacks that they live in. They're tired, they're hungry, they're cold, dirty."
Then I probably reinforced the description by having street urchins try to sell tchotchkes to the PCs.
I don't need to describe what they are wearing, or what the shacks look like, or where everything is in relation to everything else. That description is enough to get across everything I want to say for the entire planet, and it sets up the stakes for when the Empire comes to invade.
There are times when you want to go a little overboard on descriptions, though. Maybe when the PCs are going over their loot after a really tough fight and have just nabbed something awesome, or a PC makes an awesome attack or casts fireball for the first time.