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For the most part, I am ok with their stat line. They have a pretty much standard line for a Space Marine. Ld 8 is pretty crappy, but then again, they are not people persons. They do need two Wounds or a beefed up Toughness. In the Deathwatch RPG, Techmarines are tanks. They have higher armor and toughness than any of their other brethren. The 2+ armor save is cool, but as an IC they could be easily picked out by a power weapon and killed.
The Blessing of the Omnissiah rules lets a Techmarine repair a weapon destroyed or immobilized result on a vehicle. That sounds great, right? The truth of it on the table top (at least for me), is that this means that you either have to cluster your vehicles around the Techmarine or forget about it. As it most often turns out, the vehicle that needs it is too far away from the Techmarine to do any good. In addition, it is an extra roll and for many Codices it is a crappy one.
I suggested before that this roll should be turned into a cover save. If you are within 12" of the Techmarine, you get a roll. 5+ plus one for every servator and a reroll for the full harness, is fair. This speeds things up and does not require you moving your Techmarine all around. There is precedent for this with Ork Mek's Kustom Force Field.
Another thing that is disappointing about the Techmarine is their weapon options. Don't get me wrong, the full servo harness is awesome and fear-inspiring option but the rest is pretty bland. What they need is modified versions of their weapons similar to what the Flash Gitz of the Orks have. For a few points they can make their bolters twin-linked, give their power weapons +1 Strength or +1 Attack for the thunder hammers. This is completely supported by the fluff as Techmarines often modify their weapons. They certainly modify their armor.
The thing that really drags down the Techmarine for me, is that they are essentially a 50point kill point. You have to surround them with some ablative wounds (I prefer Scouts to Servators due to better survivability and weapons but that is neither here nor there) or risk losing them to stray fire. Then the Techmarine does nothing for that squad.
Taking the last idea I had of having the Techmarine modify their weapons and applying it to the Chaplain template, you get weapon modifications that affect the unit. The Techmarine should have to be attached to the squad before the start of the battle and lost his ability to break off +/- his IC status. Then, any modifications that the Techmarine takes also affects the unit's wargear. The points cost for this should be between 10 and 20 points depending on the options.
Joining the Techmarine to a squad like Warlocks or Necron Lords short circuits the cheese of having him join multiple units and modify their wargear too. The cover save can make things a little beardy but I think the speed argument and not needing to have him in base contact are worth a little potential abuse of that rule. Also, you could play it off like Fabius Bile's modification of CSM and have him modify 1d3 number of units with a random bonus.
Any of the ideas here have the massive potential for abuse and cheese but then again almost everything does. I think that everything is fluff-based and withing the spirit of the game and rules. I think that it will speed things up and give the Techmarine choice a place in my army. I also think that it is going to make them a 150+ point Elite choice, but that is how the dice roll.