The techmarine with full servo harness model is one of the coolest space marine models around, but the only real time i would get a techmarine in my army is if its for fluff than anything else. Maybe in a Iron hands army with lots of tanks, 3 techs with servitors and stuff like that, lead by those Tech commanders with an extra wound. Just my input though, so do what you want.
Techmarines are good for two things, one is repairing damaged vehicles, however the odds are so against you and the conditions required to try are so hard it's generally not worth it. The other thing you can use them for is a makeshift devastator squad. Get a bunch of gun servitors, and give them some nasty guns, you can only take one plasma cannon but that's okay. The tech marine has 2 special abilities from his special equipment. The first is called the Auspex. This little gizmo means that if anything deploys (via infiltrate, deep strike, whatever) within 4D6 of him or his squad the *whole squad* gets a free shot at it. the other thing he has is a signum which allows a re-roll of one missed to hit roll. So... roll a 1 on that plasma cannon, miss on a multi-melta shot, do you really need another hit from that heavy bolter to break that orc mob? Roll it again and get another go!
I tend to view it as a Devastator squad you can get in the Elite section. It can't get lascannons but you get meltas so there is the anti-tank, and you can get a bunch of heavy bolters and a plasma cannon so that's plenty of firepower good for killing anything you want to point it at.
However, you can't fluff them up with extra marines so hits will immediately reduce your firepower, and the save on a servitor is only a 4+ not a 3+. And while the servitors are cheap, the guns on them aren't. But you get the option to 'snap fire' on new units if the opponent isn't paying attention, isn't expecting you to use them, or scatters badly.
Many a Tyranid player has tried to ambush my Devastator squad with Lictors, Raveners, and other forms of unpleasentness, only to get ambushed by my Techmarine in turn. Sweet revenge.
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