First OS trip to NZ, DS was 6 months old, for two weeks. It was easier than driving to East Gippsland with him for a weekend!!
Second (REAL trip!) was to Ireland and London (just London, whereas Ireland was a road trip to see family and attend a wedding), DS was 19 months old, we were gone for 3 and a half weeks.
DS was great on the flights, though BFing is the biggest reason for that. He became crap in the car, cos he wanted to be running around, not stuck in a car (at least in the plane he could walk up and down the aisles!). He loved train travel in London (and the bit we did in Ireland).
I think any age for kids is great to travel, and the less gadgetry/equipment you take, the more enjoyable for everyone. Much easier if you can share a big family bed whilst away, especially for booking accommodation We co-sleep, so it was with curiosity that B&B's would say "So, there are 2 adults and one child...and you only want one double bed?"..."ummm, yep". With two children, maybe you could get them to sleep in the one bed whilst away, or keep one child in with you, so that you dont' have to book as many beds. After all, it's a travel holiday - it doesn't have to be the same arrangement as at home, does it?