It depends where you are plannig to go. In Africa i start using it, but I took it out. In Asia I am not using. When is really mud the wheel doesn´t goes as it gets stuck. But in asphalt in Europe will be good for winter.
Plastics are better but more noisy and less efficient.
Metals are the best

I always used mudguard, so I don't really know what it is biking without them. Don't your feet get soaked when biking in the rain without mudguard? Or your panniers covered with mud on dirty wet roads? I agree when it is really muddy mudguards are counterproductive!  I would recommend aluminum over plastic as people do complain about plastic ones breaking. Mudguards are necessary when there is more than one person traveling, otherwise the one biking behind get covered with anything on the road.

I have a stainless steel set that was originally off an Apollo lady's 3-speed. You never know when you're going to get caught in a downpour. Even though Australia is a fairly dry climate, when it does rain, it comes bucketing down. Sydney has twice the annual rainfall of London, but has a lot less "rain days".

The mudguard is one of those things that you don't "need" all of the time, but when you do need them, you'll be glad that you had them fitted. Also, I agree with ensuring that there is plenty of clearance between the mudguards and tyres, as mud, sticks, dirt, etc can clog them up and if there is insufficient clearance, you can break stays, guards, or just make your trip very hard going.