The Technomancy Discipline is themed around manipulating vehicles and their machine spirits. This manifests both in buffing your own vehicles and weakening or damaging enemy vehicles.
Subvert Machine is the primaris power, and is a Malediction that targets an enemy vehicle within 18". When cast, you randomly select one weapon on the target and roll off against your opponent. If your opponent wins, nothing happens. In the event of a tie, you get to fire Snap Shots with that weapon. If you win, then you get to fire the weapon normally. I hate to say this, but this power is absolutely terrible. You have to cast it, then hope you randomly select a decent weapon on the target, then you only have a 41.7% chance of firing it normally. You need to be able to choose the weapon or have some advantage on the roll off with your opponent to make this worthwhile.
Blessing of the Machine is a WC1 Blessing that targets a friendly vehicle within 24". The target ignores Crew Shaken and Crew Stunned and gains Power of the Machine Spirit. If the target already has Power of the Machine Spirit, then it gains +1 BS. Handily, if the psyker is on a transport, he can cast it on the transport. This is a useful spell as it provides several benefits, but will be most useful in that it allows you to prevent an important vehicle from being stun locked. It's not amazing but it's a serviceable utility power.
Machine Curse is a Focused Witchfire that inflicts D3 S1 Haywire hits on an enemy vehicle within 18". This isn't bad at all, especially as Space Marines really don't have access to much Haywire. The range is a little short but it's definitely worth firing at enemy heavy vehicles.
Reforge is a WC1 Blessing that targets a friendly vehicle within 24". It allows the Librarian to pretend to be a Techmarine, either repairing a lost hull point or a weapon destroyed or immobilized result. In addition, the target gains It Will Not Die for a turn, giving it the chance to regain another hull point. It also allows the psyker to cast it on a transport they occupy. This is a great power for keeping your tanks in the fight, and its range and ability to repair multiple types of damage make it very flexible.
Warpmetal Armor is a WC2 Blessing that targets a friendly unit within 24". If the unit is a vehicle, it receives +1 AV to all armor facings. If it's not a vehicle, then all models in the unit gain +1 Toughness. This is very useful for all of our tougher vehicles, particularly Predators and Land Raiders, though it won't help against grav, haywire, or gauss. It also provides a nice increase in durability for Centurions, Bikers, and even Terminators.
Fury of Mars is a WC1 Assault 1 18" Beam with Haywire. If you can line up a couple of vehicles, you can reliably damage them all. However, you won't be destroying any of them with a single shot, so this is more useful for chipping away at multiple vehicles. It's a decent power when combined with the rest of your army's shooting, but it's not amazing on it's own.
Machine Flense is a WC2 focused witchfire with an 18" range. It targets an enemy vehicle, which loses D3 hull points. For each hull point you take off, you can inflict D6 S4 AP6 Rending hits on an enemy unit within 12" of the target (you can divide this up between multiple units if you take off more than 1 hull point). This power does a little bit of everything and is capable of doing substantial damage to a vehicle while also damaging nearby infantry. It requires a bit of lucky dice to do maximum damage, but Machine Flense is a strong power.
Overall, I think Technomancy is a good but very focused discipline. It contains 3 offensive (2 good, 1 decent) and 3 defensive (2 excellent, 1 good) powers, though only Warpmetal Armor affects something other than vehicles. The only real downside is the extremely weak Primaris. If you are taking lots of vehicles in your army (such as an Anvil Strike Force or Fist of Medusa Strike Force), it is definitely worth considering.