The Warpsmith is the chaos analogue of the techmarine. Corrupted and mutated by the taint of the warp, the Warpsmith is nominally responsible for the creation of monsters like the Forgefiend and for the maintenance of the old legion's war vehicles.
Armed with a flamer and melta gun, the warpsmith can be deadly to both troops and vehicles, with a little effort with positioning. His shatter defences ability is an interesting one that can help decrease defences of enemy strong-holds. He can repair vehicles (a modified roll of 4+) to remove immobilized, repair a HP, regenerate a weapon, and can curse (gets hot!) enemy vehicles. I find it interesting in the description, that he only gets the modified roll of 4+ if he has his mechatendrils intact. Does this mean we're going to see more future models being able to remove specific weapons from enemy targets? Intriguing!
So what upgrades might be good for a warpsmith? Well, I don't think that additional ranged weaponry (which consist of combi-weapons or a plasma pistol) are particularly needed given the in-built melta-gun and flamer, plus hand-held bolt pistol. There are a few potential options, but I actually think a "naked" warpsmith is a good option. Load him up in a rhino with a reasonably sized squad and plough in behind defilers, vindicators and the like. Repair them as necessary and target any stray enemy ground vehicle that gets too near. Or sit him back manning a quad gun on an Aegis defence line even, to take advantage of his BS=5.
But let's try a few different builds beyond "naked":
Warpsmith, Mark of Nurgle, Aura of Dark Glory (140 points)
This one is simply buffed for a bit more survivability. The role here is to repair, snipe and cause a headache for enemy support. Sit next to a Forgefiend (or similar) in the back field perhaps and keep them running? Swapping the Mark of Nurgle for a Mark of Tzeentch would also be an idea with similar goals to this build.
Warpsmith, Mark of Khorne, Aura of Dark Glory, The Murder Sword, Gift of Mutation, Veterans of the Long War (185 points)
Right, I know I said that Gift of Mutation was risky, but what the heck. This build sees the Warpsmith take on a much more pro-active role. Pack him in with a close combat squad (terminators perhaps?) and head straight for an enemy leader. On the charge (and facing the nominated target of the murder sword), I believe that's 6 S8 attacks at AP1 (2 base, +2 mechatendrils, +2 charge). And if the enemy is a space marine, that's going to result in a roll on the Chaos Boon table pretty quickly. Along the way, he's going to be sniping at enemy vehicles and generally causing a nuisance. Pity about only having 2 wounds and T=4, but that's the risk here. Feel free to swap to a Mark of Slaanesh to strike at I=5, its all good!
My only criticism with the warpsmith is that we can't give him a daemonic steed, or a bike to ride on -- that'd be very cool. Otherwise, I think this guy actually has some potential.