凄い - terrible, dreadful, terrific, amazing, great, wonderful, to a great extent (EDICT)
Sugoi is probably one of the most misunderstood words in
Japanese. In Japanese 101, most students learn to use it in describing "
cool" things, like
Inuyasha or
Shiina Ringo*. Its usage isn't limited to good things, though; you can also describe a
train wreck or a
hurricane or
9/11 as
sugoi, and
Japanese people will not be even slightly
fazed.
"So how can the word mean something good and something bad at the same time?" Actually, it really means something along the lines of "incredible," and can be used for things that are incredibly good and incredibly bad. Bizarre things are called hen, or possibly okashii, but not sugoi. (chinakow suggests "awesome" as another good translation)
Macho men are more likely to shout sugê na! in a ridiculously deep voice; the standard sugoi is more common in the dainty vernacular of schoolgirls.
So now you know, and knowing is half the battle.
* I'm not saying that these things are cool; I'm saying that Japanese 101 students think they're cool.