I started using one at about 9w with jenna asshe was very chucky, and I wanted to sleep her oon the side. We kept it in use untilwe stopped wrapping her at about 9m or so.
I found it great, I dont know how well they are recommended for SIDS, but Jenna slept in our room til 6m, and so I found I slept with one eye open on her anyway.
I also found that by about 6m Jenna was rolling over to sleep on her stomach between the rolls (but she was a very early active baby) but by then I didn't stress as much about it all.
I wouldn't use one unless I had a reason too - a chucky baby sleeping on their side is a pretty good one.