I believe this to be a better series than Saki because I am a big fan of 'the underdog'.
Sure, an argument could be made that Saki itself is a series about the underdog because Kiyosumi High has never made it to the nationals; but let's just look at their all-star lineup: a infamous middle school national champion, an aloof president who is friends with pros, an airhead prodigy who defies common sense, and an energetic midget vanguard who stacks up points as long as she's dealer. Infamy aside, the fact remains that all its members has been playing mahjong for a good while.
On the other hand, Achiga-hen is a school made up of quite literally nobodies. None of their member has received any critical acclaim or achieved any level of play that received media attention. In fact two of their members hasn't played mahjong in years. Their roster isn't full of players with strange powers and other crap that only rich schools with arrogant mahjong-club presidents could possibly afford. In fact, they are comparable to Tsuruga in that they are the odd school out, that nobody in their right mind could have predicted would make it to the semi-finals of the prefectural qualifer. Individually they couldn't possibly hope to make it very far in the world of mahjong. The key focus being that they made it to the nationals through hardwork and teamwork, and not just through individual merit.
If I had to point out the bad qualities of this series I'd have to say that it's too fast paced. I really would've liked to see them focus more on the road to the nationals, but I assume because of how far its counterpart is along the author can't afford to waste any more time on a prequel setting. They will not make it to the semi-finals, but I still feel like rooting for them. That's what makes a great underdog series.
Actually, maybe I only like this series because they remind me of Tsuruga Academy...