No one likes airline food, and vegans usually don't have the chance to complain as we often end up without food. Here's my experiences of the airlines I've been on.
No trouble getting our vegan meals, although they did try to find a special vegan kids meal at first, which was cute (no they don't have one for vegans!). Had the meals ready without a fuss both ways, the food was great, if the same both times. But some serious thought went into it, the most I've ever seen. Like sunflower margarine instead of butter. Like a small bottle of extra virgin olive oil as well. Like a replacement croissant for mid-morning meal, which was a dairy-free, organic croissant so that they were 100% sure it was vegan! Like I said, lots of thought went into every last detail.
Aer Lingus can do a reasonable vegan meal when they remember. You usually have to pester them regularly before finally boarding (at booking, a few days before, the day before, at checkin) to stand a good chance of getting your meal. They almost always give you a hard time claiming you've only booked a vegetarian meal or no special meal at all. On more than one occasion I've been asked to produce my boarding pass as proof that I booked a vegan meal, only to annoy the flight attendant more when she discovered that I had booked it! Still, if you complain in writing (ask for a feedback/complaint form on board) about not receiving your meal when you have booked it they often give you a €30 voucher in return.
They have Darina Allen advising them on their meals, a "famous" chef in Ireland, so they should have some idea. Generally not too adventurous, and always with the standard milk, butter and cheese accessories. But okay if you do get it.
Surprisingly good food and no hassle when we flew Czech Airlines. Hot vegan food (almost a first for us!). Hash browns and fried beans. Exactly the same meal both ways. Staff are unbelievably dour and rude though, part of the experience I guess.
KLM Dutch airlines had the best vegan food of an airline I've ever had. Hot food, spiced black bean dish plus salad and fruit. No difficulties getting the food. Main difficulty was the cheery wakeup at 5:30am for breakfast!
Kirsten writes Qantas and Rex and even Royal Tongan Air, supply very good vegan food normally. I have not really had any problems - they have always provided interesting meals - breakfast or dinner... etc. A few things I found to be strange were though when all other passengers got a fresh fruit salad for dessert I got a packaged fruit in syrup! bit odd and on another occassion everyone was offered cornetto icecreams for dessert and i got offered a box of raisins - hello!!!