Having just finished this after a few hours of gameplay, let me share my thoughts.
The game opens with one of the most unforgiving levels in a game, ever. Even on the lowest difficulty (on which I played), I had to attempt getting through it several times. You start off virtually unarmed, while your enemies are armed. Deal with it. It gets better later on, but the difficulty is still rather punishing - if the lowest difficulty wasn't a breeze, and it wasn't, that probably means the game is a bit hardcore. Either that, or I'm actually devolving in my FPS skills.
Outside of being a specifically themed (gangs, the city, street violence) game, Kingpin has three mechanics which have been rather innovative at the time of its premiere (1999). The first of these is the fact that you can have up to two companions who will help you out, and you can issue them limited orders (Follow or Guard, essentially). This would potentially work, if not for the very high mortality rate of the companions and their limited combat abilities. Sure, they can stand their ground, but a lot is down to chance. Sometimes they defend themselves for several minutes, sometimes they fall down right at the beginning of a battle. You can never know. This isn't a very innovative mechanic because Half Life did something similar before this game, but I figured I could mention it.
The other two mechanics are connected to weapons. The first one is the fact that if you do not have your weapon holstered, NPCs will react to a drawn weapon - either by cowering, or being aggressive, more often the latter. In itself, this is an ok idea, but the implementation is terrible. There's only a handful of situations when it pays to not have the weapon drawn, and the later you are in the game, the less sense it makes. Also, all it takes is to hold a crowbar or lead pipe out - so whenever I wanted to crack open a crate or box next to an NPC, I was attacked by that character. And putting the weapon down doesn't solve things - they will attack you until you're dead. I had to kill more characters than I wanted to because of that.
Finally, you can buy weapons, ammo, armour and upgrades at shops called Pawn-o-Matic. A good idea, but unfortunately, again, poorly implemented - there's not too much stuff to buy in the end, and the upgrades make no sense: there are 5 (or so) for the Pistol, another single one for the Heavy Machine Gun... and that's it. Why did no other weapons receive upgrade options? Why can you upgrade the Pistol, the weapon which gets useless pretty much after you get any other weapon? No idea, but it's textbook bad game design.
The weapons aren't anything special. The most interesting one is the Flamethrower, but unfortunately, despite being very impressive visually (same goes for rocket and grenade explosions, those look great despite their age), it's a weapon which ends up being insanely overpowered when the enemies use it (get hit and you lose around 50% of armour and Hit Points, no randomness here), but rather shit when you use it. I could never get the range right (it only hits at a certain distance), and while I was trying to douse an enemy in flames, there was always someone else pounding me to the ground with different weapons. Most of the weapons are criminally inaccurate, which makes fighting at long ranges rather tedious.
The level design is interesting because, and this is mostly true about the initial levels, and becomes less so the further you go, it's somewhat open-ended. There's a certain thing you have to do to progress, but you can navigate the location at your own leisure, of course taking into account the enemies. So you'll get some absolutely optional room or buildings to explore, and some alternative paths to take. Don't be fooled though, this is no Deus Ex.
The graphics engine has aged poorly, though I still have a certain fondness for the Quake 2 engine. As mentioned before, fire effects (explosions and the flamethrower) are quite impressive and nice to look at, but the character models are pretty much ugly and blocky. The environments are all kept in the same vein - brownish-grey streets and factories, with some overtones depending on the location.
Sound is alright, though nothing in any way worthy of mentioning. There's a lot of swearing, including phrases such as "F*** You, You F***!" etc. The music is all taken from a Cypress Hill album (or so I think), and it fits the setting, and some of the songs are even nice to listen to. However, the choice of tracks is very limited, so it gets boring rather quickly.
All in all, Kingpin isn't something I'd recommend. It was enjoyable enough, but just doesn't offer anything that other titles don't have, and has many flaws which most modern games don't anymore. Comparing this game to the terrific Soldier of Fortune, which was released only a year after Kingpin, it shows that the authors mostly wanted to hype the game with the unpopular theme and controversy. The gameplay doesn't offer enough to be very enganging, but I guess that if you're an FPS fan like myself, you might want to check it out. You can get a digital copy from Good Old Games (gog.com). Regular players and people who don't generally enjoy FPS games have nothing to look for here.
Final rating: 5/10. Very average game overall, but enough to be somewhat enganging if you're in the mood for some shooting.