Once you start this game, you'll find it incredibly difficult to want to stop. Not only are the graphics quite well-made and the scenarios somewhat inventive (I mean, what other game has you getting shrunk down and avoiding getting trampled by comparatively colossal girls?), the run-and-gun gameplay is some of the tightest I've experienced, probably on par with FPS titles from earlier in the decade.
But we all know why you're on this site. And on that note, the porn in this game is comprised primarily of femdom and 'reverse rape'... which might not be to everyone's tastes. What separates this game from most others, however, is that the punishments you're dealt for getting caught in an attack are only slightly more harsh than a slap on the wrist. In other words, Harem Trigger doesn't lock all of its erotic content behind failures and 'game overs'. Quite the contrary—after beating each of the game's four bosses, you're rewarded by being allowed to watch your avatar be affectionately tended to by other girls in various poses. They're a sort of "unskippable bonus" in that regard, as you absolutely need to view them in order to move to the next set of levels.
In terms of gameplay, your character's motions are very fluid, though jumping can be a little bit finicky. (Thankfully, you're not required to jump very often aside from a couple of times in the de facto tutorial stage, 1-1.) Every gun you're able to purchase has its own unique feel, with even their 'Custom' versions having slight differences from the normal models. The range of obtainable weaponry is quite varied between SMGs like the "Sucorupyon", high-powered semi-auto rifles like the "N-Strike", shotguns such as the "SUPASU", a rocket launcher modeled after the M202 FLASH, and even joke weapons like a karate chop and a literal hand gun. Every weapon varies in power, speed and function, which makes playing certain levels much different.
The only negative point I might have to reward this game is its gross and obtrusive mosaic censorship, going far outside the bounds of what it needs to obscure... but the only time you're ever stopping to really pay notice to it is in the aforementioned "bonus levels" after beating any given boss. All in all, this game is a definite must-play, especially for enthusiasts of the FPS genre. CQC Software went above and beyond to make this game play and feel as comfortable as possible. The sound design was handled greatly and none of the sound effects or music ever get boring, and visually speaking, the game has some well-rendered models and each of the girls has a distinctly unique feel about them.
Now, if you'll excuse me... I'm grabbing my sniper rifle and going to play out the boss battle against Nagi for the millionth time.