My buggy experiences in the past six months or so:
Singapore Airlines, in ZRH, SIN, AKL and SYD: no problem at all, 5 or 10 minute wait, collected and delivered with a smile.
Easyjet, in GVA and various UK airports: no problem, friendly service, 5 minute wait at most.
Swiss, in GVA and ZRH: they collect it on boarding OK, but on arrival nobody has a clue what has happened to it. Spent 30 mins trekking around GVA being sent in circles by clueless cabin crew, ground staff and baggage handlers. Got the buggy in the end, after much cursing.
Air New Zealand, in AKL: utterly useless, unhelpful and incompetent. Made us check buggy in as hold luggage - when we protested, said they would lend us a buggy to get to aircraft. Once we had reluctantly parted with ours, it turned out there were no loan buggies available... well, my wife let 'em have it with both barrels, and they eventually found a buggy. In the meantime I quizzed the check-in supervisor about the AirNZ "no buggy" policy, as I hadn't seen anything about it when booking. It took her 20 minutes to find it on their own website - I asked what hope the poor customer has of finding the info, if senior customer service staff can't? She looked at me like I was talking martian ("I don't design the website", she pouted). They promised us another loan buggy would be waiting for us at disembarkation in SYD - it wasn't, of course.
Another AirNZ staff member explained something similar to what TightSlot mentioned about injuries carrying buggies up and down stairs - but, honestly, this has to be bollocks. All parents (some of whom are presumably airline employees) carry buggies up and down stairs constantly. The typical travelling buggy weighs 7 or 8 kg - i.e. about the same as any other piece of carry-on. SQ, EZY and Swiss staff handle them without complaint and apparently without injury - are they supermen/women?
BTW, for long haul travel with babies or toddlers, I heartily recommend SQ: cabin crew were outstanding - helpful, patient, efficient, always one step ahead of what our baby wanted/needed. In contrast, AirNZ might as well put up a "families not welcome" sign.