If you play stealthy, I find DE:HR to be a mix between Splinter Cell and MGS. Maybe even a little rogue/thief Skyrim play style, most only b/c it's first-person when you're not against a wall so you find yourself manipulating sight lines and moving insanely slow. In terms of playing as a shooter, I'd describe it along the lines of getting raped, because that's what happens to me about as soon as I am discovered. Without upgrading your... self... you are incredibly weak, and as previously stated the enemies have insane accuracy.
What I regret is I specifically heard "You might find yourself upgrading only stealth and then find it totally useless in a boss battle" in a GameSpot review and yet I completely ignored that and did it anyways and now I'm stuck. I'll be able to get past it, but the shooting mechanics just aren't super awesome and moving around cover is a little slow to do in a firefight with a guy who literally is constantly moving towards me.
The visuals are nice. They used this sort of, half-baked SSAO on the Xbox version which makes things look a little muddy at times, but it's still SSAO and makes rooms look have a little more depth. If you're playing on a nice PC I'm sure you'll fair better, but this is one of the less lazily developed multiplatforms out there. The character models are OK, we're not talking about any kind of insane polygon count and the ears could use some work, but aside from volumetric lighting and SSAO, I think it's the visual style that is appealing over the actual technical achievement.
I appreciate the fact that they made the game versatile so you can play guns blazing on stealthy, I just wish you could change between third and first person at will, because I would play a lot more in third person since I am going for no alarms and no kills, which means I'm barely ever shooting.