My only problem with this, to prevent perfection, is the foreground (or are they midground?) like trees/buildings, which are silhouetted. Its a great effect, but I would rather more of them in it, such as taking from a lower angle to get more silhouette against the sky, or a higher angle so there is none.
Just my opinion.
you place it in fron of your lens, it allows you to darken the sky so that you have to overexpose a little - that way you can get some details of the ground. basically it 'evens out' the brightness, so that you can expose well on all the scene. an example would be this photo: [link] - without the filter the ground would have been mostly blacked out.
since you have an slr, you can use it - you need a filter holder that screws onto your lens, and the filter itself. the most common type is cokin, you can get good prices on ebay! look for cokin P system