Despite what everyone said in Helios comments (tl;dr), I believe Sentinels were introduced to assist Tenno in their missions in some way.
One would take a wrong turn thinking that it is made to increase your DPS (or potency). I believe Sentinels are chosen based on one small ability that makes your grinding easier (because what else you do in Warframe, amrite? :D ).
Dethcube: Vaporize, deals quite amount of damage and stuns (if not kills) first enemy who gets too close to you (or when you get too close to the enemy). Also, DC
have the greatest attack range trigger of all Sentinels, and comes with a hit-scan, spin-up rifle, which is effectively modded with Fired Up. However, the big drawback is that Deth Machine Rifle uses Primary Weapon Mods.
Carrier: Vacuum, picks up stuff lying around for you (health orbs, energy orbs... stuff! nuff' said). Carrier has the shortest attack range trigger of all Sentinels, but it comes with the Sweeper weapon, which behaves like shotgun (and uses shotgun mods) so it is always "Firing for Effect!". Sweeper has the most powerful damage of all sentinel weapons, even after DE nerfed it because of it was clearly too strong. If you mod it well - it will steal almost all your melee kills (don't use that with Galatine xD ).
Djinn: Fatal Attraction, draws enemies close. To be honest, I don't have this sentinel, but from what i gathered from multiple sources, it doesn't "draw" enemies, just aggro them, so they focus their attack on sentinel. Useful? Sure, why not, as long as it survives machinegun/laser barrage. Also, after "attracting" enemies to itself, it activates his trap card, and after roughly 5 seconds it stuns and damages every enemy in range for... 150 unknown damage... Oh, and if suddenly required amount of enemies would be too low: the skill just goes off for 30 second cooldown. I say: useful, why not, but I would expect more from a sentinel made in goddamn laboratory and costs goddamn arm and leg. It also have very long range trigger,
but it's not worth it, it is almost as long as DC's.
@edit: Stinger range is around 50 meters. I don't know why it has such long range, but I believe putting well-modded Deth Machine Rifle* on this would be tremendous. Why not stick with the Stinger? Yes, it deals much more damage than *DMR. However, Stinger projectiles have travel time, unlike *DMR. And maybe it can hit those slackers 50m away from you, but overall, won't be such effective.
and finally, most recent, shining and striking...
Helios: Investigator, scans your enemies, using your codex "ammo". At first, I thought this was a great idea. A sentinel that makes your life easier. Just like an automatic vacuum cleaner. When I got it I was dissapointed. It doesn't scan unless you run close to the enemy, let him damage you for over 6 seconds or until you see that "Scan XP" over him. Then you can kill him. Hope they buff it up.
And it comes with a Deconstructor ranged-melee weapon, which behaves like a glaive and uses the same mods. Sometimes I like to watch my sentinel making moral ponder about if it should kill an enemy or take a picture of him first. All I want to say, is that once that guy is leveled up, it goes back on the shelves, because it ain't no better photographer than I am, especially when I Bastille my enemies. @edit: copy'n'paste from comment
Deconstructor - short range (like sweeper) fully modded for melee damage yields about 55 impact damage (that's right - impact again) but most notable thing: 25% status chance! Biggest status chance of all sentinels! This is great. This is what should sentinel weapons do! Not damage - status! This way it would be awesome. Too bad that Hello has short attack range...
To sum up: Ghost is for survivability with/in stealth, Wyrm is for survivability w/o stealth, Carrier is everyone's friend (except grineer... and corpus... and infected... did I mention corrupted? oh well), Djinn is a Fatal Mistake and Helios will be fine if Investigator gets buffed.
And to answer your question: Are Sentinels OP? Orginally I made this blog so you can visit it after reading the title (should have called it 58008 or something...) but there you have it: No, sentinels are not OP. They just try to be helpful, and some of them fail at it, while others do their job right.
Sentinel weapons on the other hand? That is a story for another blog! (spoiler alert: Thumper does deal a lot amount of damage close-range)
Thank you for reading and have a good day!