The only class really worth having a high dexterity with are the thief classes, simply because you get 5% extra to many thieving skills with each extra point of dexterity.
AC bonuses are as follows:
15 - -1AC
16 - -2AC
17 - -3AC
18 - -4AC
...
21 - -5AC
...
24 - -6AC
25 - -6AC
We also get reaction adjustments and missile attack adjustments with high (and low!) dexterity. This means that a character with high dexterity usually gets to strike earlier in a round (this is also modified by the weapon the character is using, a warrior wielding a two-handed sword will typically strike after the warrior using a dagger).
The missile attack modifier is fairly self explainatory, if you use a missile weapon a high dexterity will improve your odds of hitting. The bonus starts at +1 to hit at dexterity 16, and parallels the AC advancement (i.e. +5 to hit at dexterity 24 and 25).
Hope this helps
