Well after reading most of these posts i thught id share my experiences as a sentinel player.
First off, i am not a very experienced mmo player. I played wow for probably a month, and i managed to get a few characters to level 30 but i couldnt seem to break the 30 mark. So, you could say that im a bit of an mmo noob, a hotkey game noob anyway. When i came to tor i had no idea what rotations where, had never played a melee character before.
Anyway, now my actual experience with sentinels. I started out with the combat tree up until about level 20 something. Up until this point, i was finding it average, but not easy to get through combat. I really realised that i wasnt doing very well when i was at lvl 17 on coruscant. I challenged a lvl 15 sentinel with a watchman spec and got beaten. Thats when i really realised i wasnt playing this class properly. I wasnt happy with the combat tree at that time anyway so i respecced to watchman. After that, everything just became so much easier. I think combat is more of a responsive play-style and watchman is much more rotational. I set up a good rotation and the game really came alive. I breezed through nar shadda at level 19 solo.
I think its really just about finding a playstyle that suits you and the way you want to play, and honestly, i am really loving my sentinel right now and breezing through content.
Anyway, just my 2c.