Let’s examine the Pathfinder Tiefling racial traits, alternative racial traits, favorite class abilities, and feats for your player character (“PC”). There is some good stuff and no level adjustment for playing them.
Tieflings Racial Traits – they are all the rave for Dexterous, Intelligence, or Dex/Int stat builds (alchemist, investigator, inquisitor, wizard, magus, witch, rogue, and archers to name a few). Not only do you get a bonus in those two stats, but you get Darkvision, Elemental Resistances, and bonuses to Stealth and Bluff.
Alternative Racial Traits: You also have great alternative racial traits. I like Pass for Human so that you can use Enlarge Person and other humanoid only spells. If you are not going the Pass for human route, I like Maw or Claw, Prehensile Tail, and Scaled Skin for the appropriate type of builds. I also like Prehensile Tail to hold Rods while leaving the hands free.
Favorite Class Abilities: The Favorite Class Abilities for Alchemists and Paladins are universally great. Some of the other favorite class abilities are great if the campaign focuses on outsiders.
Tiefling Feats: Unfortunately, the only Tiefling feat worth taking is Armor of the Pit.
Variant Tiefling Heritages: Now, Tieflings are really all the rave if the Fiendish Heritage feat is waived by your GM – as it should be. The Variant Tiefling Heritages cover a variety of stat bonuses that can be applied to optimize a wide range of PC builds. The spell-like abilities of variant heritages are also great in many instances. See this Free Paladin Tiefling Guide.
Conclusion: If Tieflings are allowed in your campaign, strongly consider them as your Dex, Int, or Dex/Int PC race. If Tieflings are allowed and Fiendish Heritage is waived, strongly consider them for any PC build.