R.W.
I used the ergo all the time from when my son was about 4 months (when he'd out grown the Moby wpap) until he was about 20 months (when I was preg with #2). I really carried him in it ALL the time - never really liked pushing a stroller and he seemed much happier to be carried. The ergo is great because the weight distribution is so much better than the other carriers I tried - I could have used it even longer, his weight wasn't an issue, just my growing 'bump':)
As for the leg spread thing - when we first started using it I had the same concern. It looked a bit awkward because his legs wern't long enough for his knees to bend at the sides and they just kind of stuck straight out. But he didn't seem to mind, and it certainly didn't cause him any pain or damage to have his legs spread like that - he crawled/walked/and now (exclusivly;) runs just fine.