This is a new type of praetor for the Iron Warriors, and therefore has access to all the regular bonuses such as rites of war and so forth. On the negative side, they're worth an additional victory point if an enemy takes them out. But their bonuses are neat: stubborn, Shatter Defences, and the potential to take a servo arm as well. The Shatter Defences special rule is kind of neat, but feels like it should have been just a touch more powerful - it reduces a cover save of a single terrain feature by one. I wish it was either more than one terrain feature, or the cover save was reduced by more than one. Either way would have made the Warsmith much, much more attractive. That said, the Warsmith is an attractive choice for the Iron Warriors and one that is also really very fluffy!
Here are some builds that might appeal:
Warsmith, terminator armour, servo arm, thunder hammer, iron halo (200 points)
An extra attack from the servo arm at S8 is very attractive to say the least for this build. The idea here is to place with other terminators in a land raider and go forth and conquer.
Warsmith, servo arm (135 points)
This is one to team up with other techmarines to ensure a vital piece of equipment is kept in place during the game. He's a bit naked, so keep him safe at the back of the board, but will provide access to whatever rite of war you like. Cheap and cheerful!
Warsmith, Cortex Controller, thunder hammer, melta bombs, iron halo (170 points)
This is one to keep near a block of allied robots (Castellax and so forth) whilst being embedded in some kind of larger troops or heavy support infantry unit.