All 4 of my babies have had walkers, from the first one 16 years ago to Jack who is 11 months. They have improved a lot in safety over the years, but it is just something that you have to use with adult supervision. None of my kids have ever had an accident in them....but I'm sure there will be stories of kids who have.
As for development..I don't know all the ins and outs but I suppose it like anything..as long as its used in moderation it should be fine. It certainly didn't harm my kids. The first two walked at 12 months, my third walked at 10 months and Jack is now walking at 11 months. None of the kids have ever had any problems with feet/legs/back etc.. the oldest girls are 16 and 13.