In a 4th ed Apoc game, my squad of 3 bright lance walkers took out an ork stompa in one volley. All six lances hit (guide) and each one penetrated. Most amazing thing I've ever seen in a game of warhammer. My 270 point squad of walkers took out a 650 point titan. Of course, the walkers were scrapped on my opponent's next turn when he decided that he didn't want to loose any other vehicles to them (15 lootas shooting 3 times each eats through a lot.)
I'm a big fan of walkers, though I rarely use them. In smaller games where I know I won't have to worry about a ton of tanks and things I'll take them. But, if I know there's going to be a lot of points for anti tank weapons, I probably won't take them. Granted, it doesn't take much to kill a walker...
~edit~
I just realized that my post doesn't have anything to do with the topic of the post, so I'll add a bit more than a cool story.
I don't think that the walker's role has changed much. Starcannons kill most anything, and with 12 shots, you can expect 5 or 6 kills from the walkers. It was that way in 4th, and I think it's that way in 5th as well.