If you notice a person whom you haven't seen in a while has lost weight, should you mention it?

The way to mention it without being overt and possibly insulting about it would be to say something like "Wow, you look great!!" and leave it at that.

It's always safe to say, 'Great to see you again! Wow, you're looking fantastic!"

Even someone who's worked hard at losing weight and is pleased with her result doesn't necessarily want people saying "Wow, you've lost so much weight and you look great" because that can ONLY mean "You used to look totally gross".

If I notice any weigh tloss or weight gain on someone, I never mention their weight. NEVER.

Howsoever, if I notice weight loss, I would say something like "you look great" without pinpointing weight.

I usually would not, because my preference is the opposite of yours- I don't enjoy unsolicited conversation about my weight loss. So I'm treating others the way I'd like to be treated. 

I did make an exception when a friend of mine had lost so much weight that I wondered if it might be illness. I approached her out of earshot of others and asked her, "This weight loss, is it because of good health or illness?" This person was someone I used to be very close to, which is why I felt it would be permissible to ask and uncaring to not ask.

People are probably hesitant to mention someone else's appearance these days for fear of coming off as harassing or creepy. 

Weight loss, a face lift, dental work -- best to not get too specific when someone has a make over. "You're certainly looking wonderful, better than ever" is a strong complement that avoids hinting at overweight, jowls, or bad teeth. 

I would say that the person looked good. 

depends on why they lost weight

choko's right. (Usually is.)

It's considered rude or invasive to mention someone's weight, even in a positive way. Saying someone looks good is still fully acceptable.

Yes, I sure would................