The core book states that the Logis Implant talent is:

The character’s logis processors work with preternatural speed, filtering sensory input and allowing the character to analyse and anticipate the actions of those around him. By using the character’s Reaction for the Round, the character may make a Tech-Use Test to make use of this Talent. The character gains a +10 bonus to all Weapon Skill and Ballistic Skill Tests until the end of his next turn. The character must pass a Toughness Test when he uses this Talent or gain a level of Fatigue.

Am I missing something or is this the worst talent ever?

You have to spend your reaction and succeed two rolls to get +10 for one action. If you fail the toughness roll, not only does the -10 from fatigue cancel your bonus, the penalty lasts longer than one turn. If you fail the Tech-Use test, do you still have to make the Toughness roll? If so, you can end up immediately giving yourself -10 from fatigue and still have spent your reaction.

What?!?

You must roll Toughness test even if you fail Tech-Use, yes. But it's really hard for tech-marine to fail Tech-Use test and even harder to fail Toughness test, and in one turn you can make a lot of attacks with appropriate modes. Not so bad for 500 XP.

Has there ever been any clarification on -when- you use the Reaction to activate this talent?

RAW, Reactions cannot be spent during your turn, so arguably, wouldn't you be able to activate this ability before the start of a turn, thus allowing you keep your Reaction until the last moment, and then using it if you didn't actually need it?

EDIT: Also, its been ages since I've played DW. Don't Astartes have some form of immunity towards Fatigue? I'm certain some of the implants make the Toughness test to resist it easy, but I could of sworn there was some mechanic stating that Fatigue doesn't even affect them (until its just too much or something).

RAW, Reactions cannot be spent during your turn

Well, since DH1 you can exchange your reaction for something in dozens of different ways.

About Fatigue - they will not collapse before TB fatigue levels, that is a lot, but they receive -10 from first level.

A space marines ways of removing Fatigue are as follows:

- You can spend a fate point to remove 1 level of fatigue.

- Triggering a demeanour with some good RP should allow you to remove all fatigue.

- Fatigue goes away with time. Each 10 minutes of normal rest removes one level of Fatigue. A full hour of rest will remove all levels of Fatigue. - Normal people (from other rulesets) remove them at the rate of 1 per hour and 6 hours to remove them all.

I was sure that pain suppressant did something for it, but looking at my PDF, apparently not!