Endless Waltz picks up where the Gundam Wing TV series left off, so I recommend that you watch the television series first as it does deal with war and politics very well. Endless Waltz still makes sense if you haven't even seen the series, but it will spoil plot developments if you intend to watch the series.
Gundam Wing was an excellent series, but the final episode was very disappointing, Endless Waltz tries to make up with this and manages to suceed and fail. The television series had a well developed plot which actually forced you to watch every episode or you would be totally lost in who was fighting who, and why etc. Endless Waltz is more out and out action, but does clear up some issues with the original project meteor campaign.
On DVD Endless Waltz is presented as three episodes, quite why these were never included as part of the Gundam Wing series is a mystery, but with new upgrades to the Gundams and especially the Tallgeese, I am happy. My DVD is the original Jap version, so all the bad language and unedited violence is still present. This makes little difference as you have seen the Gundams destroy enemies hundreds of times before.
The music for Endless Waltz is absolutely great, from the background music during the movie and the ending music during the conclusion and the credits, very good.
The battles in Eternal Waltz are superior to those found in the television series, the new Gundam designs are great and Relena's kidnapper is a great suprise, which makes sense to those who have actually watch the series and the strange relationships that have been going on. Without detailing too much information on the plot, I can say that Eternal Waltz is a great addition to the Gundam universe. If you like Gundam Wing then get this, but please watch the series before hand, otherwise you will never forgive your self.