Levequests are really popular for crafting and gathering classes, which (without having the guildhests/fates/dungeon leveling of combat classes) take longer to level. If you're interested in getting into crafting and/or gathering, I'd save most of your leve allowances for those.

If you're not interested in crafting/gathering and will only be pursuing combat class(es)/job(s), the combat levequests are a fairly decent source of XP, not as fast as either FATEs or dungeons, but faster than guildhests (with the possible exception of the first time you run a partiuclar guildhest as a particular class/job since there's bonus XP for that). It's nice to throw in a few from time to time. Personally, I tend to find levequests a bit more relaxing than FATEs (which can get hectic if you keep running from one to another) or dungeons (where you've got a team of other players relying on you), so I do them when I'm tired and just want some simple solo-able leveling content.