It's a very fast and efficient way to level it. Especially for those leveling ranges that hardly have any dungeons that are worth repeating after you completed the quests once (esp. 52-58, 76-100), they nerfed dungeon completion XP fairly hard a few weeks ago which makes them pretty pointless to repeat.
From 90 onwards (if you can fly) just doing the bonus objectives in the areas is quicker than leveling over invasions, but doing the invasions and saving up on the crates will give you instant ilevel 700+ when you ding 100.
Yet its a very convenient way to level alts, you just have to log in every 4 hours and go afk twice for 15 minutes.
The most efficient way to use the invasions is with a friend who can carry you (well obviously three friends that can summon you are even better) using the 300% XP potion. You have to group up, then get as quickly as possible to the invasion. As soon as you reach it, the one with the xp potion can log out until the friend says that the stage will change soon, then you log in again and this way you can skip the tedious first and third (and major chunks of the second and fourth) stage saving precious potion time. You cannot do that alone, if you log out too long you will be placed into stage one again.
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That's not true, at level 50 you make at least two levels with full heirlooms. It's starting to decrease from level 60 on where you will make about a single level per invasion.
Did it with three toons the last weekend:
http://eu.battle.net/wow/de/characte...rimmeat/simple
http://eu.battle.net/wow/de/characte...eckibus/simple
http://eu.battle.net/wow/de/characte...Snowpaw/simple