I have a hard time coming down firmly on either side of this question. I do not think that workers are always best served when they are in a union, but I also feel that unions do have some place in the world.
To defend this statement, we can say that firms can easily abuse workers who are not unionized. We can see this with firms such as WalMart. These firms can pay their workers low wages and can have them work in relatively poor conditions. From this point of view, unions are beneficial because they ensure that their members get good wages and cannot be abused arbitrarily by the management.
However, it is also clear that unions can be very harmful to workers. Some unions become too demanding and end up hurting the very people that they are supposed to protect. For example, if a union demands too much in the way of wages and benefits from a manufacturing company, that company might simply pull up stakes and go produce in another country. This would cause the union members to be worse off than they would have been if they had had non-union jobs in the manufacturing company.
Thus, there are clearly arguments to be made for both sides of this issue.