Is it just me or is the sentinel just not a very good class? You would think the sniper rifle would do some crazy damage with how hard it is to hit with, but it can take up to 10 hits to kill some enemies. Any tips on how to play this class and not hinder my team?

I haven't been very impressed with the Sentinel and I hope Hi-rez revisits them in an update. I definitely view them as the weakest class at the moment.
Since I'm crap with the sniper rifle, I only use the Sentinel as a mess around class when I want to be quick on my feet during base defense.
  
Claymores are a good augment for guarding the Gen or flag. If you place them on an indoor slope lazy infils/etc zooming in usually don't see them and glide over them to their deaths. Since you have a bit more movement you can replace them a bit faster than say a Doombringer's mines, and with the right perks/unlocks you can have x4 of them down at a time. They're also great for corners.
The secondary pistol is a bitch to aim (I believe it isn't hitscan) but it does good damage (350ish) and has a decent clip size.
The jammer is also great to hide around the Gen to help vs infils and cloak other defenders.
 
If you're good at sniping then you can be very useful at picking off pathfinders with the flag, but other than that I view traditional snipers as free melee/sticky kills.
Sniping deployables from afar is also an annoying but effective tactic.
 
So in summary: Motherfucking claymores everywhere at all times constantly.

I like using the Juggernaut

the Sentinel seems like a really high skill cap class, cause of the way the sniper rifle works. If you miss, it really hurts, cause the way the rifles do damage is by using energy, or waiting for it to charge up, meaning that constantly firing shots is really bad. That said however, fully charged shots don't feel like they do enough damage for how hard it is to hit people.

Setting up claymores around the flag is hilarious though, i ended up with heaps of kills when i first played sentinel, and i didn't even hit a single person with my sniper rifle.

The Sentinel secondary is pretty damned good. I can't remember what it is called, but it is a pistol that has exploding rounds and ricochet bullets.

I have better luck when I'm not too far away from whoever I'm shooting. There's quite a bit of dropoff in damage if you're far away from your target.