Not having twins myself but from what i have heard:

Cons:
Much bigger/heavier tummy during pregnancy means you have to be much more careful what you do/get more tired/more painful muscles/
More complicated and longer delivery
Almost double expenses
Double the work (especially as if one wakes up and cries the other probably will as well)

Pros:
They will usually have a very strong bond and keep each other company.
If you take the costs for 2 separate babies, you end up paying less if they both come at the same time.
Don't have to go through pregnancy/delivery twice (if you wanted 2 kids).


At my monthly checkup today we met a couple in the waiting room who had just had twins (they were visiting the pediatrician next door). They said it was a lot of work and often they found they didn't have enough hands for everything. Each twin had to take turns for everything like changing and breast feeding that made life a little difficult.
However they were also really happy that they had 2 beautiful babies (one boy and one girl) and got it out of the way all in one go .

I would say (from what i have seen/been told) twins is kind of the same as 1 baby (a lot of work, effort and stress... and for some reason totally worth it), but more... of everything .