Is the product I hear ads for all the time, promoted for flushing out the nose with saline as a health maintenance strategy, a meaningful advance? Does it have a downside?

Unfortunately, we do not see this as having practical benefit for the vast majority of users. It is something that is simply promoted to manage a nuisance that is normally self‑limiting, the accumulation of mucus in the nasal passages. There may be certain individuals who need extra help to avoid a possible obstruction, but that will not happen for most people under normal circumstances. The human being was designed to function despite having such a technological innovation at hand. It is much akin to the past fad of subjecting children and others to a regular enema to flush out the distal colon. Such a procedure is not needed except in extreme cases threatening an obstruction due to fecal impaction. The same is true of nasal lavage. Moreover, there is a potential downside in flushing away defensive chemicals being elaborated by the mucus membrane, which is the first line of defense against respiratory pathogens. While there is a buildup of organisms, those are normally handled by defensive chemicals being secreted into mucus by the cells lining the airway. While the buildup of foreign organisms lodging in the nasal passages can be removed by lavage, it will also wash away that chemical barrier, at least for a time. So there is a trade-off here; it is not a given that there is an advantage in doing so.