Unless they fixed it in LH, the 25% chance doesn't work. The AG always gives you 1 xp per season. It could still use a buff though. For low level heroes, it is kinda nice. After a certain level, however, the xp bonus is far too low to be beneficial.

There's only a 25% chance they will fix this since we've been asking for it to be fixed a long time...

don't forget that you also get +1 Fame per turn for an Adventurers Guild since LH. I agree that the xp bonus for stationed champions is still pretty underwhelming and needs some love (I think the best solution for this problem that has been argued for a long time now is xp reward for each constructed building in the city), in the meantime the fame bonus alone is enough to warrant an Adventurers Guild.

I don't know how many times I've suggested this, but I personally use THIS adventurer's guild:

<ImprovementType InternalName="AdventurersGuild">
   <DisplayName>Adventurer's Guild</DisplayName>
   <Description>Champions gain 2% of a level per season when stationed in a city with an adventurer's guild.</Description>
   <RequiresCity>1</RequiresCity>
   <BarredTerrain>River</BarredTerrain>
   <BarredTerrain>SwampTerrain</BarredTerrain>
   <PreferredTerrain>City</PreferredTerrain>
   <PreferredTerrain>Category:Land</PreferredTerrain>
   <PreferredTerrain>Forest</PreferredTerrain>
   <Prereq>
      <Type>Tech</Type>
      <Attribute>Heroes</Attribute>
   </Prereq>
   <LaborToBuild>840</LaborToBuild>
   <GameModifier>
      <ModType>StationedUnitStatBonus</ModType>
      <StrVal>UnitStat_Experience</StrVal>
      <StrVal2>OnlyChampions</StrVal2>
      <Value>2</Value>
      <Operator>%</Operator>
      <Provides>Champions gain 2% of a level per season when stationed in a city with an adventurer's guild.</Provides>
   </GameModifier>
   <GameModifier>
      <ModType>Resource</ModType>
      <Attribute>Fame</Attribute>
      <Value>40</Value>
      <Provides>+40 Fame</Provides>
   </GameModifier>
   <AIData AIPersonality="AI_General">
   <AIPriority>5</AIPriority>
   <AITag>CommandPost</AITag>
   </AIData>
   <ArtDef>Art_AdventurersGuild</ArtDef>
</ImprovementType>

Changes from the .80 version LH Adventurer's Guild are:

1) Cost increased from 540 to 840.

2) 1 per faction REMOVED, can be built in every city. Due to the cost, you would generally only do this in important cities worth governing, beyond the first one built.

3) Gives a flat one time +40 Fame instead of +1 fame per season (to avoid stacking +1 fame from repeated buildings).

4) Gives 2% of a level per season instead of the useless 1 xp. Every 50 seasons a governor (or any stationed champion) in a city which has built this building will level. This can be easily balanced by adjusting the % bonus.

In a 700 turn game, if you miraculously built it on turn 1, despite not having the tech, and production... that'd be 14 levels. However, in the REAL game, if we assume you beeline the research for this building (Heroes tech), and then build the building as a first priority... I still think you'd be in the 100+ turn region, likely later, and you would have handicapped yourself severely by doing so. The numbers might be up for debate, but I am certain that this building is the solution that commander (governor) heroes need to do their job.

In my personal play experience, this building encourages me to level my low level heroes in the field, where they will gain much more than 2-3 xp per turn, and then retire them to important cities as managers / roaming patrol defense around the level 10 point where the percentage gain is advantageous compared to campaigning the field trying to pick up stray monsters. They still weren't better than a company of  plate mail armored lancers built in my main essence fortress, but at least they weren't utterly useless.