When I was young I believed they were good as it was what I was taught and fell in the socialist-leaning mentality of much of the country.
Having started to learn about economics, entering the industry, and seeing the arguments many people make for unionization have definitely led to me to swing much harder to the "unions are evil" side of the argument.
That said I do believe they are a double edged sword of sorts. When the only employer for a certain profession is the government for example (i.e. not driven by profits and losses, or not a part of a competitive market) unions can actually help make things fair and get a point across that otherwise would not be visible. But in general they are much less efficient than profits and losses in a competitive market and in those situations will lead to a decline in growth that you would otherwise not get.