I can't say I'm a big fan of flight in D&D. Of course it belongs in the game somehow, seeing how it's a staple of the fantasy genre as much as the superhero genre, so I defend it's place in D&D, but in all my right to contradict myself, I don't want to see players flying around all the time either.
The big problem with flight is that it's effectively a form of immunity. As soon as you're able to fly constantly, you've made yourself immune to melee attacks from earthbound foes (by staying out of reach) and to the disadvantages and hazards of difficult terrain (by not touching it), both relatively common elements in a D&D game. The DM, knowing the players are effectively immune to such things, will have to rule anything that relies on that, from countless foes to interesting terrain-based encounters, out simply because the players would reduce that kind of challenges to next to nothing simply with the advantages granted by flight.
The same goes for Freedom of Movement and possibly Death Ward. Those things've been the death of grappling and anything negative energy-based since the dawn of D&Dkind..
Basically, anything that makes the characters immune (directly or by consequence) to relatively broad groups of challenges should be rather pricey... If for nothing else, then to make sure a characters doesn't end up immune to anything but divine retribution... That is, until they make a magic item that gives you immunity to that as well.