I bought cheaper, bulky over-trousers early in my walking career, but I found them awkward to put on and take off and far too warm much of the time. I decided to go 'upmarket' and, despite what seemed an exorbitant price at the time, took a chance on the Berghaus Paclites. They turned out to be worth every penny and they're the ones I carry when there's even just a chance of getting uncomfortably wet legs.
Like Paul, I like waterproof trousers and have a pair of Rohan Dry Frontier trousers. For me to choose to wear them though, the forecast must be for fairly continuous rain or bad weather. They feel like lightweight winter-lined trousers; they're very comfortable and I like having easy-to-access pockets (handkerchiefs are important to me in the cold or rain) and the Frontiers have six! However, unless it's a cold day, if the sun comes out I start to get too warm in them - and, unlike over-trousers, I'm stuck with them for the duration.
I'm someone who much prefers to be too cool rather than too warm and I hate being sweaty. So, if the weather is just drizzly or if the rain is heavier but intermittent, then I'm happy in my normal walking trousers that are 100% nylon and which dry extremely quickly.