If you're going martialist, then blade breaker is by far the most powerful ability. The furious focus line is good as well, and is a solid complement to any melee build. For crimsonate, I think the PF line is the standard tactic. Rend is still bugged so I don't think its worth it. For Red Hand just max out the bloodling line and the bloodling buff. Crimson mark has a small range and its only marginally useful, but its just one point so get it if you want I guess. Summoners typically get the bloodling + buff skills then pick a damage spell. You can pick the imps, but their damage is significantly worse than bloodlings. All 6 imps you get at max rank = 1 bloodling. You're better off picking a spell like blood sleet or blood command.
Stat-wise, if you're not going hardcore, don't worry about vit. If you go summoner, most people go all wisdom (I chose to go all str with my summoner so that I could wield better melee weaps--you need a corpse to start creating bloodlings and its very difficult to do this using wis weaps). For any other bloodletter build, you should go all str for max dmg. You really don't need dex for +hit since you get attack rating bonuses from skills and equip.
Originally posted by: Canai
Well, I guess there are no viable builds after you have maxed the passives for marksman (gadgeteer) - I'm level 24 or 25, and I'm only as good as my guns. Points into other skills are useless unless I have 30+ points (15+ levels) to put into them. Shitty.
For gadg, you should go for the piercing barrage line after you max out your passives. The alternative active ability to PB is napalm. It got nerfed in the last patch but it still does solid damage. None of the widgets are really worthwhile at this point since they move slowly, but the galvanic widgets can be good for boss fights. In the grenade tree, noxious grenade is nice since it scales well (does %hp damage) but it still kills slowly, so it'd work better as a side ability to go with gun damage.
Keep in mind that gun damage was nerfed, dex's affect on gun damage was nerfed (1% -> .5% dmg per point), and also all +%dmg mods on weapons were halved on new items. The net result is that there aren't many good guns out there. Depending on your difficulty I can give you a prepatch gun, does 175 dmg requires 133 dex.