I'm not getting the complaints about the Sentry Gun protection. Maybe because I'm on the 360? We get Sentries to last two hives, easy. But, then too, I'm talking about running coop.
The Seeker protection definitely works. On a first hit, it's as if the Seeker was not even there. I've knifed several in a row, flashing red, and survived. My bro laughs and says I'm crazy - and he's the one that takes risks, not me.
I kind of concur that the 5% extra damage on everything does not seem worth it. Where that seems to come in most handy, though, is as someone already noted - on the Rhinos and other heavies. This is especially noticeable when you have Armor Piercing maxed out. We've had noobs and even some experienced players freak out over how fast we take down Rhinos using the combo.
I haven't unlocked any of the 10 teeth or higher items yet. Just the 5 teeth items and, as a whole, they seem to give a little help. I think anyone looking for some sort of invulnerability, though, is going to be disappointed. But, then, I don't think these things are meant to make you invincible.
I usually have tok many skill points by the end of a match but they are crucial at the beginning.
I also think the medic and weapon specialist upgrades are awesome... The medic especially for helping new players escape POC for more teeth. I run around waiting for the second barrier to drop then run by and instant revive the newbs and regenerate my health quickly.
I think the biggest waste of teeth is on the start with a locker key perk.
It not a waste for Nightfall or Mayday. I love starting with a key since they can be very rare (especially on Mayday).
I always upgrade pistols to carry two primaries, so then I get to keep the gun from my locker, if it's one I really want to keep (LMG or Hone Badger, for instance).