I remember this series from my childhood in the mid-seventies. At the time it was still futuristic and very high tech. Yes, this has aged and some of the plot now seems slow, but I believe this is more down to a comparison with what we are used to now than a true reflection of the series itself.
For many years I waited for it to be repeated and always remembered the sound of the UFOs themselves distinctly. It puzzled me why there was an American leader and the left hand drive cars drove on the right hand side of the road when it is based in Britain (which drives on the left and has right hand drive cars). Also puzzling was the token American in a British-based and operated organisation...and although this was never explained, I cannot imagine Cmdr. Ed Straker as a Brit or being played by anyone other than Ed Bishop.
For all its links to the marionette / Supermarionation world of the previous Anderson concepts, UFO stands alone as the first series to bring the idea of a repeated alien incursion on Earth and for showing strong characters that one can relate to on a personal level due to their flaws and idiosyncrasies. It was a series I enjoyed in the 70s and again this year when I found it and watched it after nearly 50 years! Sure - some nostalgia on my part, but a lot of good episodes showing character development, human strength and weaknesses and covering many topics that are taboo today (thanks to Political Correctness).....and all without a computer to create the special effects.