I kept most of the guns I've found since I upgraded them early on. But since I play stealth, I end up hoarding them, then never using them until boss fights (WHICH FUCKING SUCK, BY THE WAY. THEY REALLY FUCKING SUCK.) I played a lot of MGS so stealth with a peashooter is something I'm used to. Though I do crack a good share of skulls along the way. My favorite map so far has to be the one where you sneak into the base leading up to the first boss. There are a lot of great little moments there.
As for upgrades, I haven't upgraded past 4, which I've still only seen a handfull of. Haven't relied on cloak as much as I'd like (this isn't TF2
) but the icarus parachute upgrade is VERY handy for shortcuts, and the line of sight radar is nice.
There was one mission where I was in a whore house. I snuck into one room and a guy and a girl were on the otherside of the doorway. The guy heard me and shouted. I barreled through the door, mashing on Takedown hoping to knock him out before he shot me. Except the whore ran through the door first and ate a knuckle sandwich right to the face. Then I was out of energy (WHY DON'T THE OTHER BATTERIES RECHARGE???) so I had to shoot the other guy. I heard someone coming my way, so I dumped the bodies (yes, even the unconscious hooker) off the balcony and parachuted down after them.