By reading in the description "Recruit a gang of thugs and rule the streets" or "If a new gang member turns out to be a punk, blast him, and make room for new ones" gives me the sensation that this has something more than simply straight forward FPS that sounds some kind of micro-management, but i don't know... is this a simple linear shooter ?

The storyline is very linear, but the game did have a few interesting ideas to separate it from other shooters at the time. The first is the feature you mentioned, where you can hire thugs to run with you. Basically, you have a couple of communication keys in the game, one for "positive" and one for "negative". You can talk to most characters in the game by choosing to hit either of the keys and they'll react accordingly. There are characters throughout the game that if you're nice to them, will join your gang for a certain amount of cash.
Once they're in your group, you can have them attack your enemies or hang back, but your control over them is fairly limited. It's really lightweight, nothing like what you'd find in the Rainbow Six games.
The other interesting thing is you have to buy most of your weapons / ammo and you can also buy modifications for your weapons (for example, a silencer for your pistol). You get cash by finding it on the bodies of people you kill, which makes for an interesting twist. You can either let "friendly" NPCs go freely, or kill them to take their money.

sounds interesting, thanks !

Have to say how flexible this game is with regards to resolutions. I have it running at 1600*1200 (which seems to be the games max resolution) although if you have it in full screen mode it is set off to the left of the screen. In windowed mode it can sit centrally but you can't mouse out unless you return to the menu and I suggest taskbar set to auto hide (XP)
I'd hate to think how the censors would have handled this game today. It seems much more in your face than the GT series as far as language and violence is concerned.
It was never a 'great' game but it is cool to return to and I remember having fun with it when it was first out.
If you don't mind a pretty liniear shooter there is fun to be had here. I'm looking forward to getting a flamethrower although my memories of screaming burning gangsters may be my mind playing tricks.

You can hire thugs to fight with you and you can buy stuff from shops RPG-style, but most of that is near the beginning of the game; you can even do simple "quests" from time-to-time and talk to NPCs in different tones of voice. It's unfortunate that they didn't keep that vibe going throughout, as the game becomes more of a 'straight shooter' the farther you get into it.
Still, it's one of the most visceral shooters around, even today.

I finally got to play this as my PC back in the day would not run it. Im not very impressed with it and it is very linear. The whole thing feels tacked together rather than the awesome game it was supposed to be.
The whole hiring thugs thing was no more clever than getting the security guards in Half Life to follow you.

When did 'linear' become such a dirty word?

When the designers decided that players didn't like opening doors. ;)

IIRC Kingpin was intended to be a bit different than it ended up being, but the game is still good.
The thugs are extremely well implemented, IMHO. Their A.I. and general balancing is very good. It's easy to take for granted just how well they do their job the majority of the time, but this should be appreciated, because it's a hard thing to do, and Xatrix did it well, where so many other games make you feel like you're teamed up with a bunch of dim-wits.
Admittedly it's a while since I played the game, but my recollection is that my hired thugs not only kicked ass, but bailed me out of trouble on many occasions, and yet despite all that, I never felt redundant.

If there was anything about Kingpin that made it stand out from the rest of the crowd of FPSs, it was the damage model: in a time and place where you had 100 HP and once that was gone you were dead, Kingpin had bodypart based HP, location-based damage models, and weapons that felt deadly. The AI was par, the dialogue cheezy and rife with missed opportunities, and the gameplay itself was rushed but passable. But I will never forget playing it way back when and going "Wow, headshots really DO kill you quicker..."

What sucks about the Pawn Shops is that they don't have half of what they advertise; trying buying a flamethrower or machine-gun in the first two levels with your spare cash and suddenly they are all out. You pretty much have to find your equipment off of dead enemies, including health and ammo. The 'Shops are good for mods, of course. Can't get those anywhere else.
And yes, the AI in this game is killer. The friendly thugs almost never get stuck, they can climb ladders, leap across gaps and down cliffs and heal themselves over time. If a fight is too difficult you can throw them into the fire ala R6: Vegas and let them sort it out.