Love mine, wouldn't be without it and my 2.5 year old DS2 will still happily go in it.
I wouldn't think at 5 months you'd need an infant insert, but if you are planning any more it might be an investment. DS2 was 9 months old when we got ours and obviously didn't need one. hehehehehe I think it is meant for babies who are tiny and can't hold their heads up i.e. newborns.
I can wear it for HOURS, even now that he's about 13ish kg.