Psis also work better together than other casters thanks to powers like Metaconcert. This makes basing a campaign around Psions a more cohesive option than other "one trick pony" campaign ideas.
Finally, the augmentations make psis more flexible than similar spontaneous arcane & divine casters (sorcerer and favored soul).
One weakness is I always wish they would've made *some* rules for creating additional augmentations. Looking at D-Door in Complete Psi shows how many options and how varied the options can be for a single power.