Ah the Great Eagle. Very few models in the wood elf army could be considered "cheap" and yet be totally worth their points ten times over in a game.
The Great Eagle is an extremely viable Rare selection for the Wood Elf army in 8th edition. You can easily take 2 in most games and still have points left over for alternate rare selections. The Great Eagle being only 50 points provides the Wood Elf army with several options:
1. War Machine Hunters - they are 3 wound fliers that can generally take out most annoying warmachines that plague Wood Elf lines.
2. March Blockers - their ability to soar over your opponent's line and force leadership tests to march actually pays off and allows you to better position the rest of your army.
3. Mage Hunters - most lone Wizards should fear the Great Eagle
4. Rear/Flank Chargers - the Great Eagle is just strong enough to provide you with an excellent boost to your combat res. by giving you a +2 rear charge bonus or a +1 flank charge.
Anatomy of the Great Eagle -
1. Weapon Skill 5 - Against most armies these guys hit on a 3.
2. Strength and Tough 4: -1 to armor saves and T4 is nothing to sneeze at.
3. 3 Wounds - not bad for a 50 point model.
4. 2 Attacks + Stomp - again this is not a bad combat boost to a multi-charge - If you are lucky you can deal 3 wounds +2 combat res for a rear charge.
5. Leadership 8 - grats on having a Monstrous Beast with higher leadership than 80% of the armies in the game.
How I use the Great Eagles in my lists:
I field 2 Great Eagles in my 2000 point list. I use them to both hunt Warmachines and also to provide some protection/augment my Wild Riders. It is not uncommon to see my Great Eagles rear charge a unit that my Wild Riders just engaged. The 20" flight they have also allows me to redirect the entire flow of the battlefield by placing 2 units on my opponents flank and now creating a nagging worry that they have to think about during the game. I rarely solo charge a unit unless it's a cannon or a spellcaster with my Great Eagles. Their use is primarily to provide me with mobility and options during a game. Great Eagles also provide excellent bodyguard capabilities for a Waywatcher unit or a unit of Glade Guard Scouts.
I fully encourage Wood Elf players to take Great Eagles in 8th edition. Where most armies in 8th edition have adopted the "bigger is better" approach to fielding units, MSU is still an option for Wood Elves but it's an option that you have to use with near surgical precision. Don't make the mistake I did with my Great Eagles in my first few games and use them as a hammer to try to break up enemies blocks. That was foolish, they are not flying Doombulls. I recommend using one or two in your games and try using them as combat support units rather than flying warmachines (that is unless your opponent has warmachines then by all means charge the hell out of those and rid your opponent of that annoying option).
Hope this helps, now go roll some dice!