First time I post in this forum. I must admit I haven't played much traditional hex-based, turn-by-turn wargaming since discovering 4 years ago the superb wargaming engine of HTTR and Conquest of the Aegean, but the Decisive Battles series is one of those rare exceptions. I own all titles except Across the Dnepr, and I always felt the Decisive Battles system worked quite well.
But Kharkov is even better. The system has been improved and I enjoy the game even more. Congrats SSG for a job well-done with this title.
What I appreciate the most are the modifications to the Supply system. It makes much more sense to have supply go through these Formation HQ (which also provide all kinds of other support and benefits to units, making them valuable additions to the game). The supply truck system of the previous games looks more like a hassle now

The Off-Map interface that manages the arrival of reinforcements is great too. It feels realistic. I like the possibility of transiting units from one area to another, at the expense of losing some time.
Regarding the implementation of the AO concept, my opinion is rather positive. I understand the philosophy behind it and it's true that being able to move formations all around the place was not realistic at all. I must say however that playing as Germans is rather frustrating in the first turns with the AO system since you are so much limited in what you can do to reinforce the areas suffering under the vigorous Soviet advances. There isn't much you can do except weathering the storm as best you can and try to limit the damages. That's probably how it felt to most German commanders of the front at that time though. I haven't tried yet Mystery variants.
The Direct Attack system leaves me a bit indifferent, but the simplified movement system is better.
All in all, a great update to the engine !