The one problem I have with Rivalry is that it is a high risk/high reward ability and it's a nuisance when you don't get the boost and end up failing to KO at key moments.
Of course if you manage to get the boost you can score key KO's that probably wouldn't of have happened.
How about PK Gaming's boosted Haxorus? Well let's see the results:
Rivarly Swords Dance Haxorus (male) vs. Physically Defensive Skarm (Male and Female)
Haxorus @ Life Orb
Ability: Rivalry
Adamant Nature
252 atk EVs
-Swords Dance
-Outrage
-Earthquake
-Brick Break/taunt
Skarmory @ shed shell/ leftovers
Ability: sturdy
Impish Nature
252 HP/ 252 Def EV's
Rivalry Boosted Outrage (+0) does 32.3% - 38.3%
Rivalry Hindered Outrage (+0) does 19.8% - 23.4%
Rivalry Boosted Outrage (+2) does 64.7% - 76.3%
Rivalry Hindered Outrage (+2) does 39.2% - 46.1%
So that shows how much different Boosted and hindered Rivalry makes a difference between a 2hko and a 3hko (counting roost it's a 3hko with + rivalry and set-up bait with - rivalry).
It's great when it boosts but you can cost yourself the match if rivalry hinders your attack.
Of course if you manage to get the boost you can score key KO's that probably wouldn't of have happened.
How about PK Gaming's boosted Haxorus? Well let's see the results:
Rivarly Swords Dance Haxorus (male) vs. Physically Defensive Skarm (Male and Female)
Haxorus @ Life Orb
Ability: Rivalry
Adamant Nature
252 atk EVs
-Swords Dance
-Outrage
-Earthquake
-Brick Break/taunt
Skarmory @ shed shell/ leftovers
Ability: sturdy
Impish Nature
252 HP/ 252 Def EV's
Rivalry Boosted Outrage (+0) does 32.3% - 38.3%
Rivalry Hindered Outrage (+0) does 19.8% - 23.4%
Rivalry Boosted Outrage (+2) does 64.7% - 76.3%
Rivalry Hindered Outrage (+2) does 39.2% - 46.1%
So that shows how much different Boosted and hindered Rivalry makes a difference between a 2hko and a 3hko (counting roost it's a 3hko with + rivalry and set-up bait with - rivalry).
It's great when it boosts but you can cost yourself the match if rivalry hinders your attack.