Ah Techmarines. Usually these guys aren't so flash in army books because, well you rarely need someone running around repairing your tanks at such a high price. You can often buy one-two more tanks for the price of a single Techmarine and whilst he can find some use in parking lots (base to base with lots of vehicles), Elite slots are often used for more important units. There are really two armies that run Techmarine variants well. Space Wolves' Iron Priests can bring another Thunderwulf mount to the table and of course Grey Knights. We'll be looking at why for Grey Knights in this post.
We'll start with the obvious first. The Grey Knight techmarine is an independent character (unlike the Blood Angels version) and has access to the wonderful grenades of the Grey Knights. These importantly include Rad Grenades and Psychotroke Grenades (as well as Blind, Brain Mines, etc.). As we discussed earlier, these are pretty awesome and can make even mediocre units stellar in combat on the charge (looking at you Strike Squads). This is one of the primary reasons to take Techmarines. They aren't in Terminator armor either so can easily be ferried around in Rhinos/Razorbacks unlike Grand Masters/Brother Captains. The only other unit capable of doing this is a Xenos Inquisitor who is quite often cheaper and more durable with three wounds but takes up valuable HQ slots. If you want double grenade carriers and an HQ outside of a Xenos Inquisitior (you know, like all the really good choices there), Techmarines are your grenade carrier of choice.
There are other uses to the Techmarine of course. Even when taking them as grenade carriers, they do provide some excellent utility thanks to their other special rules. They come with the standard bolster defenses which improves the cover save of a ruin within your deployment zone by 1. This is great for setting up firebases that are durable and combines particularly well with Coteaz and scoring Jokerao. The Techmarine also comes with the usual servo-harness which is full of goodies and gives him some decent shooting and combat prowess. Remember though, the Techmarine only has one wound, a 2+ save and no invulnerable. Being an Indpendent Character means he can be picked out in combat and you don't want some power weapon Guardsmen striking him down.
The Techmarine also comes with the psychic powers Hammerhand and Reconstruction. Both are very useful but are unfortunately limited by the Techmarines Ld of 8. This means without psychic defenses he only has a 72% chance to succeed - decent but not reliable. Chuck in a psychic hood which is generally Ld10 and these powers may not be as useful as one would hope. When they are cast however, the combination of Hammerhand + Rad grenades is excellent and if the Techmarine has joined a Grey Knight unit, the combination of double Hammerhand + Rad Grenades is downright disgusting. That's essentially a +3 strength bonus whilst affecting the instant death threshold of your opponent. This combination is really important against T4 models as they can now be instant deathed (toughness goes down to three, base strength four goes up to six). Reconstruction is nice utility when the Techmarine is riding along in a tank or near a damaged parking lot. It makes his servo-harness more reliable thanks to the re-roll but when being used as a grenade jockey, this use will be limited.
The Techmarine also comes stock standard with And They Shall Know No Fear, just like any Marine. This isn't really that important or significant for most Marine armies but unlike most Marine armies, Grey Knights have the option to take units which do not have this special rule. These guys are of course henchmen. This means the Techmarine (and any other independent character with this rule) can join a squad of henchmen and provide them with this rule. Death Cult Assassins cry out in joy - ATSKNF and Grenades? Yay. This is a significant boost over Stubborn in many cases but remember the Techmarine is Ld8 which doesn't help Henchmen units out as they are also Ld8.
The Techmarine has a fair few weapon options as well. He has access to all of the Nemesis Weapons but with only 1A base, probably shouldn't invest in any as they aren't cheap. Remember he comes with his servo-harness attacks as standard as well. He can replace his servo-harness with a Conversion Beamer for 20 points which is decent and can provide the Grey Knight list with extra long-ranged firepower. It is quite expensive (110 points total) for a single blast but can support other ranged units (i.e. Jokerao) quite nicely and gives the Grey Knight army access to some blast weapons. He can also take the Orbital Strike Relay at 50 points which means you can take a lot of these in any given army. The OSR is pretty unreliable however since it always scatters full distance though the Lance profile can destroy parking lots and the Barrage profile has multiple bites at the apple against infantry. He can also take servo-skulls at three a piece and these should always be recommended unless you have some in your army already. The utility of being able to stop infiltration is great let alone any benefits you may gain from deepstriking or blasts near them.
Conclusion
The Grey Knight techmarine has a lot more choices than their Vanilla counterparts. Whilst not capable of taking a Servitor unit is a bit disappointing (I'll take 10 point MM servitors please!) he more than makes up for it with the ability to be a grenade jockey and this is where he really shines. The Techmarine is capable of fire support but is often a bit expensive for this role and is extremely pricey to be used as an on-field mechanic. It's a great utility but not a reason to take him.
