I honestly feel that this story is paced very well. It actually has a very realistic feel to it. You really get a sense of whats going on and Tatsumi's reasoning, strategics, and thoughts are understandable.
When I think of non-realistic soccer mangas; I instantly think of ones like Whistle!! and Captain Tsubasa... Though I suppose they have their perks. Of course, even though I'm stating that Giant Killing has a realistic plot... it still IS a manga. It can only go so far.
In Giant Killing, the plot really gets you on edge. Honestly, its really one of the few that actually get me whooping with joy when a goal is scored. For the most part, its truly unexpected; because losing is a huge possibility in this story.
Also, for once the story revolves around all the players as well as the management, fans and coaching team. Readers are told a story through the perspectives of many people, and are let in on their thoughts and emotions.
Character designs and personalities are quite likeable. There isn't one character I truly hate in this manga. The artwork is quite good, and the paneling is relatively smooth.
I have never truly been a fan of soccer before, so I can't be the best judge of whether or not the games played in this manga resemble real life pro-level soccer games.
Edit: Wow! It's been a long time since I wrote this. I am still avidly following GK! Though it's slowed down quite considerably, I still enjoy it a lot even 3 years after. I highly recommend people give this a try! If not the anime is also an excellent source as it follows the manga very closely.