I had one for years and totally loved it. Then I became very fat and could not wear it any more, so I gave it away. Now I've lost most of that weight I so regret giving my trench-coat away.
I think they're a classic that keeps coming back into fashion, and the right one will always look good.
But it has to be the right one, cut well, not skimpily. If you have lush womanly curves, then you need one that has been designed to take that into account, like a ladies' Burberry or Aquascutum. If you're less curvaceous then you can get away with a straighter (and often cheaper) cut.
I like the longer coats. Yes, generally better for a taller woman, but, again, the right cut will look good on a shorter woman. It's about the amount of fabric: too much overloads the shorter person.
A good cut will give you a lovely, clearly defined silhouette, and not crush your smart work clothes.
Trench-coats are rarely truly waterproof, but, if you get one with a good, dense weave, you can proof them at home with something like Grangers' or Nikwax's products.