Not very many stealth games can make stealth AND combat fun but Intravenous manages it. The stealth is pure Splinter Cell, only top down. You have the mouse-wheel walking speed, you have the noise meter that also tracks ambient noise. You even, for some reason, have Sam's EMP gun to temporarily disable lights. The guards are up to the challenge though: Leave blood, a body, or even an open door and they will go on alert, manually walk and tell all their friends to start doing patrols, splitting up in two man groups to randomly search the level room by room for you. Silenced guns are realistically relatively loud so you will have to rely on knives, your choking arms, and a suspiciously overpowered taser to try to get through stuff quietly.
Only two slight weaknesses. In a few situations it is tough to identify which obstacles are chest high and which block vision. You have to sometimes test with the POV mouse cursor to see but when a guard is rounding the corner you don't really have the luxury of time to check. A few later levels leave waist high garbage scattered around a full size container just to obfuscate the issue further. Second, the levels are fairly linear as a stealth game. You sometimes can brute force an alternate route but there are clearly extremely well patrolled chokepoints and clearly darker alleys and vents to get around and clearly the exit only has one path to it, but it never feels limiting outside of maybe one or two parts of the game's levels.
Combat is the other half of the game, where it borrows probably the most from Splinter Cell Conviction, but also a hair of Hotline Miami mixed with a /k/ love of firearms. Enemies are incredibly lethal, but so are you, and, while you can usually get a few, will quickly recognize you are camping in a chokepoint and refuse to rush in. Theoretically this allows you to do the Conviction style panther play of vanishing mid combat and flanking the guys who are all tunnel vision staring at where you were but the levels seldom let you pull that off as, if you left yourself a back exit, typically the enemy would have tried to flank you, or the enemies that had just been there BEFORE you went loud would have come that way too. There are situations where it works and it feels great, but in a lot of parts of the game where you want to start murdering druggies and degenerates, it is difficult to take advantage of. When it works, when you shotgun down a bunch of criminal scum and then dive into the shadows and watch all their friends run by you giving you a brief opening to sprint to better cover to reengage, it is a heck of a rush.Both styles of play are viable and you can even shift between them, but a protag loaded with armor will make a lot of noise and not be able to easily secure quiet melee takedowns, and a protag without armor will be even MORE vulnerable in combat. Most missions have armories or other ways of securing gear OSP so you have some flexibility.
That said, if you're playing as a nonlethal stealth purist, you really need to let go of that obsession. There is a cheevo for it, but some of the later levels of the game are obviously very Conviction styled, with you and guards hunting each other and objectives requiring you neutralizing all of the guards. Trying to be a leather pajamaed nerd with tranqs and magic choke knockouts all game is technically doable but it will lead to suffering if you aren't willing to at least sample the combat half of the game now and again. (And if you don't occasionally shoot things, you'll miss out on the game's incredibly groovy soundtrack.) The levels might not offer a lot of sandbox style replay, but providing both violence and stealth gameplay styles surely makes a strong argument for replay.
And if you really NEED the excuse, the game provides modding tools for campaign creation, as well as Steam Workshop integration for them, though at the time of writing the amount of people making maps is not particularly large.
Anyone who is a fan of classic style stealth games should definitely play Intravenous. It is an excellent example of the genre in atmosphere, mechanics, and style. Truly it is well worth digging into. That said, there is an entire gunplay half of the game there which you will need to be able to accept and attempt to really get your money's worth out of it.
