It depends on who is giving the opinion.
If it's someone whose opinion I trust, and who I don't think has an ulterior motive (like having feelings for me themselves) I hope I would try to listen to their reasons, even if initial way of expressing them was not very tactful. Sometimes friends or family can see things in a relationship that the participants have missed. I know that I've seen friends in relationships where I thought "You can do better than that!", not because of the person's job, or appearance, but because they were not a very nice person, or didn't treat my friend well, or were very demanding/high maintenance/needy/controlling/baggage laden/unfaithful etc. And, as an observer, I wasn't particularly influenced by their sexiness or a desire to rescue them or reform them.
If it's someone whose opinion I don't respect, I'd go with the "that's an interesting thing to say" response and bean dip. If I realize that someone is saying it because they regard my new SO as competition - I'll do the same, and distance myself from them for a while.