I hit 50 on my Sentinel and only just now think it's good, ha. I kept thinking "oh, once I get this power it'll even out" but it wasn't until I hit Incarnates that I really felt I was contributing. I built a Pistols/Dark Sentinel and learned:
-I saw someone say "a Blaster but alive" and that's about right
-Pretty solid solo: decent damage and rare deaths. Never been mezzed.
-Moderate team contribution - not enough damage to really be "the damage" on big/AV teams and not resistant enough to tank reliably.
Now that I hit Incarnate and have slotted some expensive IO sets, I can jump into most mobs and hit my PBAoEs/AoEs and take out most of them. With some support boosting my recharge and recovery I can really do some damage, but even with slotted IO sets I struggle a bit with end recovery in extended fights (I usually drop my shields fighting AVs to be able to keep mashing attacks).
I'll note that I'm in no way an enhancement expert, but I understand the mechanics and have everything "properly" slotted.
Overall I don't regret the Sentinel, but it was the first character I made on Homecoming and once I finally did make a Blaster alt I realized just how much less damage I had been doing all that time.