Currently replaying BL with Lillith, who I've played every playthru for several playthrus. The phasewalk skill was brilliant in that I can leave a fight for a few seconds, regenerate health and shields before rejoining it, melee crit someone for 835% dmg as well as be anywhere on the battlefield faster than anyone else. Always (except once) came top in Moxxi's Underdome vs my soldier and hunter friends, usually by a margin. With a "Combustion Hellfire" SMG and Firefly/Mercenary mod she's an almost unstoppable force (though I think the particular gun does have a lot to do with it :-D).
The trouble now is the new siren doesn't have the stealth mechanic (but still has some cool looking stuff, including some healing and support skills). Zero has stealth, but isn't "phased" like Lillith was so (I believe) can still take (AoE?) damage while invisible. So it'll either be Zero, or Maya. I like to appear from stealth, cause untold carnage and disappear again

Also very useful with teammates going down around the battlefield (tho apparently Maya can talent her phaselock to instantly revive fallen comrades, at range).
Saying that, it looks like they fixed the issues with Brick (sucking) in Salvadore. With dual wielding any two weapons in the game, he could well be the juggernaut of death and destruction Lillith was in the first game. I guess it comes down to how much extra damage his second gun does (100% or a different amount?), what guns his skills lean toward, how long he can keep his action skill up and compare that with the others.