My first couple games with the Defender were pretty disastrous. After experimenting with it though, I found that it is indeed a great ship.
It plays the long-range sniper game fantastically. Sitting at range 2-3 with an HLC it can do very consistent damage and is very hard to kill. Its dial helps it easily escape if it gets in trouble and can easily set itself up for another pass. A lot of people are saying that it's a "predictable" ship, but it's only predictable if you fly it predictably. It can do every maneuver in the game but a straight 1. In all the games I've played with it, I've only had to take a red maneuver twice. Even if you are stressed, you still have a ton of white maneuvers you can do, including a K-turn.
Out of the pilots, I don't like the Onyx. For 32 points you're not gaining enough over the Delta. The Delta is a great ship though. Even with it's low PS, it doesn't mind taking hits 'cause it can tank a lot. It's chances of getting blocked are also pretty nonexistant. Slap an HLC on it and it can really ruin someone's day.
Out of the named pilots, I'm not a fan of Brath. When his ability goes off he can win you the game (If you've ever slapped 4 crits on a falcon in one shot you know what I'm talking about), but you're paying a lot of points for something that's not reliable at all. Vessery on the other hand is a monster. With an HLC you're rolling 3/4 hits every shot. Combining him with Outmaneuver makes him even better. He chews through B-Wings and Falcons incredibly fast. Also Vessery won't care about being stressed, or barrel-rolling or boosting (if you have an engine upgrade). He'll get a target lock no matter what if you're managing them well.
I admit that the use of a Defender is not obvious at all just by looking at them, but if you give yourself some time to experiment with one, it is an incredibly effective ship.
The problem I have with Ves is that he requires the assistance of other ships to make him work and is just PS 6. As others have pointed out, that can be tough/costly/suboptimal to do in an imperial list.