I'm nearly done with my plathrough and as a level 57 Sentinel, I can tell you this class is extremely good... it just takes some patience to learn how to use it effectively. Once I did, other classes seemed boring in comparison (except maybe Infiltrator, which looks very fun).
One key thing to understand is that Bioware has really expanded the number of possible power combos in this game. See this thread for a breakdown - it can really change your playstyle:
http://social.biowar...index/9629730/1
The Sentinel has two primary builds that I've discovered, both of which are ridiculously effective. You can either make yourself the ultimate Adept/Engineer hybrid, capable of dealing of with any situation, or you can make yourself into an Invincible Tank.
Tank Specialization
The Tank specialization is pretty straightforward. For your Fitness power, make sure you maximize your health and shields. On your Tech Armor, make sure you maximize your damage reduction and get it up to 40% reduction when maxed out. Then choose a bonus power like Barrier, Fortitude or Defensive Matrix. If you max out the DR on any of those abilities, you will have around 70% DR, which is pretty huge.
I recommend taking Defensive Matrix as your bonus power. You can specialize that ability to regenerate your shields, so anytime your defenses are down, you can expel the barrier to get all your shields back. Between your improved shield and health, 70% Damage Reduction and ability to instantly regenerate shields, you are a serious tank and will survive most situations. Your power cooldowns wills be terrible, but the point of this build is to only occasionally use powers and mostly gun your way to victory. It works surprisingly well.
Hybrid Specialization
Personally, the hybrid specialization is where the Sentinel really becomes fun. Sure, the Tank build is highly effective, but it's a little too straightforward for my tastes.
If you read up on the power combos in the link above, you should realize that the Sentinel is unique in that it has access to more power combinations than any other class (AFAIK). Having Warp, Throw, Cryo Blast and Overload at your disposal is incredibly effective.
Because Tech abilities can now trigger combos, you now have a wealth of options. Examples of what the Sentinel can do:
Warp + Throw = Biotic Explosion
Any Biotic Bonus Power (Slam, Dark Channel, Reave, Stasis) + Warp or Throw = Biotic Explosion
Overload + Any Direction Damage Power (Warp, Carnage, Energy Drain) = Tech Burst
Cryo Blast + Warp (or any other direct-damage ability that kills the opponent) = Cryo Explosion
All these possible combinations give you massive battlefield control. There is literally nothing in the game that a Sentinel cannot take down. Keep in mind you can shoot at things at the same time, but you probably never need to.
One of my favorite combinations is the Triple Combo:
Overload an enemy to strip defenses (spec'd right, it will also stun and affect multiple enemies). Then Warp any enemy in that group to trigger a Tech Burst, which shocks an additional group of enemies in a radius. Then finish off that Warp'd enemy with a Throw to trigger a Biotic Explosion to level the group. Friggin' awesome.
When creating this kind of specialization, the key is to make sure that you are lowering your power cooldowns whenever possible. This means:
1) Choose weapons that leave you close to 200% cooldown bonus. This may initially mean just using a pistol of SMG, but eventually you can have a fine assortment of weapons. With the right upgrades, I have an assault rifle, pistol and SMG while still maintaining 200%. I also have a rifle/shotgun combo that is very effective at 190% cooldown bonus.
2) Upgrade your Offensive Mastery to reduce your weight penalty as much as possible. This again is very helpful for reducing cooldowns while giving you more robust weapon options.
3) Upgrade your Tech Armor to reduce cooldowns from 80% to 50%. This sounds like a lot still, but you'll eventually have so many cooldown reductions that it will not make much of a perceptible difference. Hard to believe at first, but it's true.
4) Make sure you use Liara's upgrade terminal and choose Power Cooldown bonuses whenever you can. I think I probably have anywhere from 15-25% cooldowns from this.
5) Upgrade the right weapons to level V as soon as you can to reduce weapon weight. The Vindicator is a highly effectively assault rifle and can take care of you in most combat situations by itself, and can easily keep you at 200% before be fully upgraded.
6) Choose armor pieces that boost your cooldown speeds. It will take time to acquire all of them, but eventually you can give yourself a 50% bonus to cooldowns through those pieces. Alternately, you can buy armor on the Citadel that gives you +30 to power damage and +30 to cooldowns, which is pretty good.
Sentinels are definitely trickier to figure out. They take some time, but once they start maturing into either of these builds, they become extremely good. I can tell you that, even on Insanity, I dominate battlefields. And best of all, they're super fun. Can't argue with that.