First, I should explain the differences between this version and the original. The endings and part of the story are extracted from the PS2 version, which got generally higher ratings than the original PC version, and paths from the Minikiss fandisc were added on (including Konoe Sunao's route), making the game as a whole feel more complete. This basically means that this is the definitive version of the VN and you should really play this one if you want to play Tsuyokiss.
Second, I bring up the simple reality of this VN... it was made in 2005, during the height of the 'moege golden age', when most of the best moege, charage, and the like came out. To be blunt, if we are just talking about variety of expression and scenario design, there are very few 'modern' moege variants that can compete with the best from the period of time between 2004-2007. This is a direct result of trends narrowing the range of expression in moege/charage as time went on, rather than an absolute drop in writing quality, so please do me the favor of not lumping this in with those 'but everything was better back then' people. I like retro stuff but I can admit when it really hasn't aged well.
Third, this is my second time playing Tsuyokiss. To be honest, the first time I wasn't impressed... part of it was that I'd just come off of Majikoi in Japanese (Minato Soft being a subsidiary of Candy Soft), and another part was that at the time I was a lot less tolerant of non-fantasy moege/charage. A few warnings about this VN is that it was made during the time when tsundere culture was just becoming fashionable as a distinct concept rather than just slang in VNs (tsundere having peaked back around 2010 in VNs, as opposed to 2007 in anime). As a result, they defined this VN as a 'tsundere' VN at the time, even though - strictly speaking - there is only one straight-out tsundere heroine (Konoe, who wasn't even a playable heroine in the original), with most of the others being variations on the kuudere (Nagomi), 'arrogant heroine' (Erika), or bakakko themes (Kani - also known as Kinu). There is also a yandere in there (most Candy Soft games have at least one borderline yandere), though I won't tell you who it is unless you ask. Basically, this was made back in an era when the various ideas of 'dere' weren't nearly as clearly defined as they tend to be now (mostly because writers basically force the heroines into those archetypes) and it is considered the work that basically defined the 'dere' idea for VNs.
Having played a lot more moege/charage since playing this the first time, I was surprised at how well it stands up to modern VNs. In fact, it leaves most of them in the dust. Part off this is that the Full Edition includes even more content than the incredibly long original (this VN took me almost thirty hours to complete), but another large part of it is that the lack of 'archetype-fitting' means that you can sit back and enjoy a much freer characterization experience. I honestly hated Kani, both the first and second time through, but I found it much more enjoyable to play through the other routes (except maybe Erika's, since she is something of an arrogant bitch no matter what point in the story you are at) than I did the first time around.
That said, this VN doesn't really compare well visually with modern VNs, save in basic aesthetic sense in the CGs/drawing (modern styles being the result of modern technology rather than actual advances in artistic skill). However, in terms of the audio, outside of a few voices like Sunao's (Sunao's voice is a bit... pale and awkward, to be honest) the audio comes out comparatively well in comparison to most modern VNs.
A few aspects nostalgia freaks and people tired of the de facto restrictions on moege/charage romances that exist today will probably like that there is a side route for the teacher, and the heroines aren't universally justified in their motivations (there are a lot fewer 'convenient' twists to character situations than there would be today, and as a result, it feels more natural despite using the same storytelling style as Majikoi).
On the other hand, this is no more and no less than a straight-out love-comedy with a lot more of the old-style slapstick introduced by Love Hina and Tenchi Muyo than you generally see today. It also shows off the beginnings of what would eventually become the 'complete nonentity protagonist' that dominates most moege/charage made between 2007-now. While Leo has a lot more personality than a modern moege/charage protagonist generally has (with notable exceptions here and there), he does show a lot of the qualities that have become cliched in modern moege/charage protagonists (half-hetareism, 'average' everything, 'is normally lazy but pulls out all the stops when needed').
So... if you want some nostalgia, mixed with VN historical value, mixed with high-spec writing and scenario design, this is a good choice... if you can get over the usual art-bigotry that seems to define all you young whippersnappers.
Edit: Oh, and otakus, it isn't your imagination. Fukahire's VA is the same one as Reinhardt Von Lohengrimm from Legend of the Galactic Heroes... one of the weirdest choices I've ever seen, lol.