As a regular marine player, I can honestly say the only techmarine that is worth including is Master of the Forge for his dreadnought, relocation, relocation, relocation skills. He isn't half bad as cheap HQ either, coming in with a considerable bunch of gear for his standard points.
Normal techmarines and indeed guard techpriest engiseers, abilities seem to be fixing tanks and bringing servitors. Neither of which are especially points efficient, you are normally better to invest the points in more vehicles, and shooting/combat servitors are quite poor in a marine army due to inferior stats and expensive weapons. In the guard, I have been known to use them as an "assault" squad, but that is on the grounds guard have nothing any good in combat