Deflation means the value of the dollar goes up. This is terrible for people who owe money, because the dollar amount stays the same but the value of what you owe has increased. It's like owing someone $10 today, doing nothing, and a year from now you owe them $100 in today's dollars. My guess is that this would have a bad effect on the economy because nobody wants to borrow. Hyperdeflation would also have dramatic effects on societal stability.
Inflation means the value of the dollar goes down. This is terrible for people who are owed money (or who have it), because the value of what they are owed decreases over time. It's like being owed $10 today, doing nothing, and a year from now you are only owed $1 in today's dollars. My guess is that this would have a bad effect on the economy because nobody wants to lend. Hyperinflation would also have dramatic effects on societal stability.
You can see from the way that I structured my post that I think too much inflation or deflation can be bad. There must be a happy medium somewhere. A small amount of inflation seems to spur investment (people will be more likely to open a business or something and go into debt) and seems to be best for the economy.
But I'm a physicist, I've never taken an economics class in my life, so you shouldn't trust my opinion as gospel.