It's funny, and entertaining, especially when you play co-op. But it's short, and doesn't have as much replay value as it should, or at least to justify the price tag. I've also heard that Fable II is better. I own both, but oddly haven't finished Fable II, so I have yet to find out if I agree with that statement.
If money's tight, or if this is the only game you'll be able to get for a little while, I would suggest renting it first. Or, buying/renting the first or second one (maybe both?) and go from there.
Still, there are better games out there with more replay value. So, it really depends on exactly what you're looking for, and your financial limitations.