We have used the wheeled walkers with all 3 of ours (8,5,2) and have no problems with their hip development...
The wheeled walkers they have now are differently designed than the ones that they had even 15 years ago... The new ones won't even go over many door jambs between rooms since they have plastic grippers on the frames... Are they safe to use at the top of stairs? NO! But thats just logical thinking... Just like you wouldn't let your crawling child around the stairs with out either you there or a baby gate just in case.
We used ours either in the basement (no stairs except for the ones going UP... And in our 1st floor where we have a door at the top of the basement stairs.
We never let our kids sit in the walkers for hours on end (like you would have to to get the hip development problems)But they are a good way for your muchkin to ammuse themselves while you are doing house things quickly... The new "walkers" or play discs are the things that will amuse your kids for hours (if they are easily amused and have no drive to venture out on their own... Wouldn't have worked for my kids at all. 3 min. and they were done with them)They lull the parents into thinking that they are "safer" than the walkers and therefore can have the kids left alone without supervision.
All 3 of my kids started walking right around their 1st birthday... (the earliest one was actually the one who spent the most time in the walker... He was the 3rd child :-)
I think it gives momma a bit of a break for a few seconds and helps stoke the munchkins curiosity while still keeping many things out of reach and keeping the munchkin in visual range.