Citi is offering a 10% transfer bonus from Citi ThankYou Points to Eithad Guest through July 5, 2017.

If you have ThankYou Points earned by a Citi Prestige, Citi ThankYou Premier, or Citi Chairman card, then you can transfer ThankYou points to Citi's airline partners and are therefore eligible for the transfer bonus.
Using Etihad Miles for Etihad Flights
Etihad Guest Miles aren’t very good for booking First Class on the Etihad A380.

For instance, Etihad would charge 88,048 Etihad miles + $183 to book Abu Dhabi to London in First Class on the Etihad A380. 

During the transfer

A few months ago I booked Iberia Business Class from São Paulo to Madrid with 42,500 Avios because of the great mileage price tag and the lack of fuel surcharges departing from Brazil where fuel surcharges are illegal. This left a hole in my itinerary between Buenos Aires and São Paulo.

Here’s an index to give my whole travel experience from Argentina to Spain some cohesion.

First Class award space in Etihad's A380 Apartment is wide open October through the end of March between Abu Dhabi and New York City (JFK).

There are two flights a day from Abu Dhabi to New York, and as of June 1 all will be flown on an Airbus A380--meaning First Class Apartments!

Through April 30, 2017, transfers from hotel partners to Etihad Guest will earn a 20% bonus.

Your best transfer options are probably between Starwood Preferred Guest and Etihad, or purchasing a Marriott Hotel + Air Travel Package that contains Etihad miles.

Normally, 20,000 SPG points = 25,000 Etihad miles (thanks to the everyday 5k bonus you get on transferring in increments of 20k), so with the current transfer bonus, 20,000 SPG points = 30,000 Etihad miles.
As One Mile at a Time pointed out, normally for Marriott Hotel + Air Package 2, you'd pay 270,000

While Etihad miles offer poor value redeeming on their own flights, there are some great deals using Etihad miles to book partner awards. Etihad miles are easy to accrue as Etihad Guest is a 1:1 transfer partner of American Express Membership Rewards, Citi ThankYou Points, and Starpoints from SPG.

An Etihad sweetspot neither Scott nor I was aware of was brought to our attention by a saavy reader who wrote the following email:

Hi Scott,

I’ve read your blog for a long while now and really appreciate your tips.

“Redeeming American Airlines Miles” Series Index

Part 1 — 5 Ways to Stretch American Airlines Miles on Economy Redemptions
Part 2 — 6 Ultra-Luxurious First Classes to Book with American Airlines Miles (this post)
Part 3 — Where to Search Award Space for All Partners
Part 4 — Award Rules and How Many Miles You Need
Part 5 — Taxes, Fees, and Fuel Surcharges
Part 6 — Redeeming for Domestic Awards
Part 7 — Redeeming for Hawaii
Part 8 — Redeeming for East and Southeast Asia
Part 9 — Redeeming for Middle East and Indian Subcontinent
Part 10 — Redeeming

The personal and business American Airlines credit cards are each offering 50,000 bonus miles after spending $3,000 in the first three months. Getting both cards and meeting their combined minimum spending requirement of $6,000 will earn you 106,000+ American Airlines miles, which you can use for up to four international roundtrips.

How to Get the Cards (and Who Is Eligible)
Meeting the Minimum Spending Requirements
Redeeming the Miles for Family Vacations, Luxurious First Class, and More

How to Get the Cards
Right now the Citi® / AAdvantage® Platinum Select®

Brussels Airlines is a partner of Etihad miles, and this is the amazing award chart for roundtrip redemptions, including 36,620 Etihad miles roundtrip in Business Class between New York and Brussels, or about a quarter of the miles United would charge for the same booking:

Brussels award space in Business Class is wide open on united.com for Summer 2017 at the moment, and Etihad miles are easy to get as a 1:1 transfer partner of Citi ThankYou Points and Amex Membership rewards, so I figured some serious investigation was in order.

How to Book a Brussels Airlines Flight

Etihad Airways is the second biggest carrier in the United Arab Emirates that flies an incredible product renowned around the world. Etihad miles have gotten easier and easier to come by as Etihad Guest is now a transfer partner of three out of the four big transferrable point currencies.

SPG Points transfer 1:1 to Etihad
Membership Rewards transfer 1:1 to Etihad
ThankYou Points transfer 1:1 to Etihad

What you've seen us write about in the past is redeeming Etihad miles on partner awards, like