Critical hits as per the book state that to make a critical hit you must roll within the crit range of your weapon. To confirm a crit you must roll within your weapon’s weapon range then then pass another attack roll. This is the official take about criticals in 3.5e. That said I have never played with a group where you had to roll to confirm a crit. In the same respect I’ve never played with a group where a natural 1 was not considered an epic failure or fumble.
These two house rules in combination add some unpredictability to the game. In any given battle there will likely be at least one critical hit or failure. A critical hit does as the book states; you apply your weapon’s critical modifier to your damage. As for a critical failure, that gets a bit more variety.
Generally speaking when you critically fail your weapon either breaks or leaves your hands and you get an attack of opportunity from any adjacent enemies. One DM recently bought a directional die, and uses that plus a d4 and your strength modifier to establish where the weapon lands. This really only works well for melee weapons. For ranged weapons a break of some sort is often the answer whether it is the entire gun or just a string. Another option is a misfire which hits a friends target if they happen to be near the intended target.
In my early days as a player, I joined a group and rolled a critical failure which hit a tentatively allied NPC who then turned on us. It essentially shifted the entire route of the campaign. While these can often shift the momentum of a battle they spice up the combat considerably, and there is no greater proof than the giddiness a player gets after hearing that he has just ruthlessly decapitated an ogre.