Many yaoi manga I have read are stale, repetitive, and forgettable. Many have crybaby, girly ukes and manipulative, personality-void semes. This is not like that at all.
One thing that I love about this manga is that Souichi and Morinaga are friends. The relationship between them isn't weak or shallow, but the two have known one another for years and have formed a genuine bond between them. I don't think I've ever seen this kind of relationship yaoi before. They aren't only lusting after each other, but can sit down and talk to one another. You may not even notice this, but that's only because it feels so natural and so right. At the same time, though, it's clear that they're not just friends either; there is a tension, one that is first one-sided, but develops gradually into something mutual.
For another thing, Souichi and Morinaga are two characters that readers can care about individually and as a pair. At the start of Tyrant, the two have known each other for five years, and have a pretty strong friendship. Their romantic relationship, however, starts out rocky. At first, Morinaga's feelings appear entirely one-sided, and Souichi denies his love with all his might. It becomes clear soon enough, though, that his feelings towards Morinaga aren't merely that of friend or of sempai-kouhei (upper and underclassman). When Souichi faces the threat of losing Morinaga, he makes a choice that a mere 'friend' would never do. From this point onwards, their relationship grows, and so do they.
The two of them are flawed; Morinaga is a nice guy, but he's still a guy. Meaning, he's horny. ALOT. Souichi is a really sweet person deep down, but he is also incredible stubborn and has the tendency to express his anger with his fists (and and his feet and his knees and....) Although the reader may disagree with them on occasions, this is what makes them real, memorable, and lovable. As the story progresses, Morinaga tries learning restraint, realizing that Souichi's feelings on matters like sex must be considered to maintain a trusting relationship with him. At the same time, Souichi must come to terms with his own romantic feelings and desires, even though they are towards another man.
Something else I really like about this manga is that each sex scene isn't the same as the last, and each has it's own importance in furthering the plot. Every time they do it, something changes in their relationship, some times more so than others. Also, every time has tension building up to it, to the point that you want it to happen. There's also something romantic every time they do it; a feeling that these two people love each other, not just lust for each other. It's never just thrown in or shallow sex.
I also wanted to address something I've seen some people complain about for this manga; when Morinaga forcefully had sex with Souichi for their first time. This is something I had trouble getting past at first, as it is a plot tool that is overused (and inappropriately so) in many yaoi manga. However, the next-morning reaction, as well as the turmoil about it for the rest of the first volume I believe dealt with the situation accordingly. I absolutely hate when the uke, immediately after the act, accepts it because they 'actually loved the seme all along' or something. Souichi's reaction is... more appropriate - for the situation, his character, and the mood of the story as a whole.
I could go on, but I'll stop myself. You really can''t understand jsut how good this manga is until you read it. And don't stop early on, because what's so great about this manga is how the relationship between Souichi and Morinaga develops over time, until the point where you can't get enough of them.
On a side note, the prequel, "Challengers," can be read before or after reading Tyrant. It focuses on Souichi's brother and his lover, and in my opinion, isn't as good. Still, you get some backstory for Souichi and Morinaga here and there, so it's definitely worth checking out - it shows how they met and some things that happened before this story's start.