I'm open to sentimentality and try not to be too cynical or stoic. As far as effectiveness, I guess it all depends how it's presented. No one wants to be cheaply manipulated. My pet peeve in this regard is overscoring.
John Ford includes sentimentality quite often. It usually seems to work. I think How Green Was My Valley had the greatest impact on me.
There might be another layer to older movies, independent of narrative – a feeling of sentimentality for the time in which they were made.