half-elves
- immune to sleep. +2 on save rolls against enchantments.
- low-light vision.
- +1 on detection, search, listen
- elvish blood. considered as elves for powers and special effects.
humans
- bonus feat
- +4 skill point at level 1, +1 each other level
the low-light vision feat may not seem really useful if vision rules are not applied in dungeon exploration, but it does allow for scouts to actually have an edge on other characters in a party.
Human have a bonus feat, but the +2 bonus on save rolls against enchantments is almost a feat (iron will). Combined to the immunity against sleep, it's good all around.
it must be said that when playing with less deeply concerned dungeon masters, those abilities will not be used often, but as a matter of fact, detection is used a lot (in the wild, to spot ennemies before they do, for exemple) [which is a part often overlooked of exploration]
overall, with abilities corresponding roughly to a bonus feat, and their bonus on skills (and automatic uses of elvish blood) they are not underpowered.
plus, roleplayingly (by the way, isn't roleplaying all this is about?) they are pretty awesome, either too young or too old, always in between.
Compared to dwarves, they do get a lot less bonuses on things, but they do not suffer any stats malus, which is great in itself.