I put off watching this for so long as the first couple of episodes didn't really grab me. Allen Walker was always what drew me to the original series, a tragic story that created a truly interesting character with the ability to see the shades of grey between the black and white driving a need to save both humans and the souls bound to Akuma. If you persevere Hallow advances Allen's character and starts to unravel some of the mysteries left by the original series.
Hallow also spends a decent amount of time on Kanda advancing his character and giving him some real depth. Well worth the watch but make no mistake, this is just a fillerless 13 part addition to the series. I truly hope they continue making the anime when the author has made enough material. Fleshing it out to 26 episodes wouldn't have diluted it in my opinion.