Core isn't. SRD can be, if you're flexible - the Unearthed Arcana class variants and psionics add a lot more variety, and if you require some of the variants (for example, Cloistered Cleric and Shapeshift Druid) it knocks out 2 of the Big 5 entirely and helps cut two of remaining 3 down to size somewhat. Wizards are still a problem, but they're always a problem. Allowing the Generic Classes for non-casters is also a significant step up in power.