For reference, an equivalent +3 Basic implant is work about 10m ISK. The one you have is quite rare and expensive and is best used as a full set of (if I recall) six implants.
Use of implants is closely tied to the use of jump clones to manage the risk.
Yes, I use a training set of implants, +4s, so about 500m ISK plugged in. That’s my risk balance in hi-sec. For a riskier position - such as a combat operation - I’ll jump to a clean clone or one with combat implants.
I don’t know your situation - how much 300m ISK represents to you - but I’d not be plugging it in as a solo implant, and would probably end up selling it. Their real value is as a full set.
Intelligence is one of the 5 learning attributes. They affect how fast you train.
Plugging in an implant with +3 intelligence or +3 memory will increase the speed at which you train intelligence or memory skills respectively.
When you rightclick skills you can see they all have a primary and secondary attribute. Shield, armour and targeting skills for example are mainly intelligence and memory skills. Ship skills and weapon skills in general are willpower and perception skills.
So if you plug that implant in, you would train shield skills (and many others - check the skill attributes) faster.
However, what you have found is not just a regular plain ‘willpower’ implant, it’s a part of an implant set, the Genolution set in this case.
Implants come in many variations. You can have plain cheap training implants, but some training implants also have extra set bonuses, which get better if you have more implants of that set plugged in.
Generally implant sets boost one attribute (for example Virtue set makes your scan probes much stronger, Snake set makes your ship faster, Crystal set makes your shield boosters much better), but the Genolution set is a bit different as it boosts all sorts of attributes.
It still has a set bonus, so it’s much more effective to plug in the full set rather than a single implant.
If I were you, I’d sell the implant you have on the market to someone who wants the full set. And if you want to learn faster, you can use the ISK from this one implant to get a full set of +3 or +4 training implants.