When I was a baby, in 1981, walkers were all the rage. I was walking when I was seven and a-half months old. Apparently, I more or less skipped crawling altogether.
I don't know about physical or mental developmental delays associated with walkers, but the dangers of my kids injuring themselves in one was enough for me to never buy one for them, and if you have carpet in your house it's pretty useless anyway. My son walked when he was 13 months old, and my daughter was 15 months which is a pretty average age for walking.
Here is a good article that discusses what to look for if you do decide to buy a walker. Things like making sure it's wide enough to not fit through a standard-size door, a gripping mechanism that prevents the walker from falling down stairs if one wheel does happen to fall over the edge of a step, etc.
If your son is 11 months old and is crawling/creeping/pulling up like he should be, then you've probably missed your window as he'll probably be walking with or without the walker in the next 3-5 months or so anyway, and there are other ways to encourage him to walk. We always had a lot of success with push-toys (stay away from the ball-popper--it will drive you crazy). Just out of curiosity, why do you want your son to walk early?