The premise of two adults playing erotic chicken until the other orgasms and "loses" is a rather transparent set up for lewd fanservice. but the value of ecchi manga has never been the narrative structure, and Gaishu Isshoku brings the heat in the areas that matter most.
Michiru is one of the hotter characters I've seen in recent manga. This is partially driven by her abrasive personality that refuses to give into the whims of a man trying to make her succumb to pleasure. Instead, she declares flawless victory over him in just about every game they play and taunts him in the aftermath. Michiru is
a very competitive pervert you see.
Her attractiveness is also driven, perhaps most importantly, by a massive bust that isn't remotely proportional to her size. One of the thickest shortstacks you'll come across in manga. The erotic scenes by which she stars in are all extremely well detailed - for example, when irrelevant male-kun cops a feel you can see each of his fingers move through the insides crevices of her shirt. This description doesn't really do the particular scene in question justice, something you'll perhaps need to read in order to really understand how easy the art makes it to form a moving visual of the scenes within your mind.
Michiru is the only character worth remembering, the guy is a totally average salaryman and that works just fine. The point is to envision yourself in the shoes of the average joe who somehow finds himself in the position to entertain a struggling erotic artist who needs to use your body as a canvas to inspire her writing. Of course you'd volunteer.
It is not a requirement for a fanservice to be funny, but it sure does help when you're trying to explain to someone why you like ecchi in the least creepy way possible. The humor of this manga is strong in a way that reminds of the overly dramatized quasi-sex scenes of Prison School between Hana and Kiyoshi. This manga does not take itself seriously at all and intends for the reader to joke about how dead serious the characters are taking a "game" that is established under dumb pretenses.
It should be surprising to no reader that this author has written hentai in the past - that is a plus for the potential eroticism of the series. But he's also done NTR and that may rear its ugly head into the narrative at some point into the future. So its best not to get yourself attached to the romantic development and treat it as a comedy first and foremost. This is a warning that may or may not come into fruition into the future. Better to be safe than sorry, though.
Gaishuu Isshoku is a fairly simple manga to judge. You'll probably know whether you like it or not by the promotional pictures. Hopefully this review convinces you in either direction.