Since the Blood and Fists days, the way I balance characters against each other is to have my players roll up characters of various levels, and throw them head to head.
To give a sense of how the mental classes compare to each other, they fight other mental classes.
For non-combat balance/usefulness and how that factors in... well that's the real trick and there we're into "art not science" territory.
Anyway, here's sample PC #1, a Powerhouse, something of a thug.
Test Powerhouse (Powerhouse 1): HD 1d10+2; HP 12; Init +1; Spd 30 ft; Defense 11, touch 11, flatfooted 10 (+1 Dex, +0 Class); BAB +1; Atk +4 melee (1d6+4, Aluminum Baseball Bat), or +2 ranged (1d4+3, Knife); SQ Melee Master; AL Gang; SV Fort +4, Ref +1, Will -1, Rec +3; Rep +0; Str 15, Dex 13, Con 14, Int 10, Wis 8, Cha 12.
Occupation: Mobster: Perks (Weapons- Aggressive Stance; Unarmed- Reactive Stance)
Skills: Athletics 4 (+6), Crime 4 (+4), Firearms 4 (+5), Influence 4 (+5), Perception 4 (+3), Streetwise 4 (+3), Unarmed 4 (+6), Weapons 4 (+6)
Feats: Armed Defense (16 Defense against melee attacks when not flat-footed), Attack Focus (Aluminum Baseball Bat), Move-By Action, Point Blank Shot
Wealth: 10Possessions: Aluminum Baseball Bat, Knife, Leather Jacket