Not easy to get there:bad roads, long hours driving, poor offers for accomodation, very high temperatures.........but really worth it! Choose the correct organization to get there. I would recommend ISrael & Magma Flow Team: great team, very pending on their customers confort. Best cooker in town too ;).
Concerning Danakil, special award for the Lava Lake: no words!
Minimum of 3 days are necessary to complete it, 4 if you want to do volcano trekking by night. Don' t miss it!