This is an easy one: if the stuff collected is rare and/or now worth more than 10-ish times the original/newsstand value of a 60's-80's era book i'll consider grabbing a collected edition.

The other deciding factor is if I just really really love a particular character and/or series whose appearances are being collected. Getting to see a full story arc or all of a character's appearances in one place/context can be a really awesome experience.

50's stuff and golden age comics are things i'll always get in collected editions just cuz that stuff usually is extra expensive in its orginal form