The group I'm in came up with a house rule that I think is kinda neat...
If your stunned, prior to declaration, (or action if an early hit stuns you before you act in a given round) you make a stun roll.
This is rolled on the MM chart. The difficulty is dependent on how many rounds of stun you currently have accumulated (and decreases as the number goes down). For 1 round of stun you use the light difficulty column, 2 is medium, 3 hard, and so on. Note that if the PC has accumulated 3 rounds of stun this roll will be made 3 times, in each of the 3 rounds.
Your roll is open-ended, modified by your stunned manuvering skill (or SD * 3, if no skill) and -100, (the GM may also modify it with willpower, importance of the action, bonuses like adrenal moves).
The number that results from the MM chart is the % of normal activity you can perform (without any stunned modifier) in a round.
This has 4 primary effects...
1. The usefulness of your stunned manuvering skill is relative to the damage (round of stun) you've received. A super fighter with a skill bonus of 100 in stunned manuvering can pretty eaisly shrug off 1 round of stun, but 5 rounds is a LOT harder.
2. There's the possibility that getting stunned will knock you off your feet, or worse. Standing around dazed by 3 rounds of stun could cause you to trip an fall... if you roll low enough on the MM chart you can injure yourself. This makes it so a buff enemy who's got 10 rounds of stun usually just falls down and knocks himself out, rather then just getting wailed on by the PC's for a hour of game time.
3. It's possible to roll higher then 100% actions on the MM chart... This can either be ignored, or a "ARRRRrrrg, now I'm angry", response to being hit. The 'Suburb job' bonuses on the chart can be similarly ignored or interpreted as morale raising results from a Character getting hit hard and brushing it off like his invincible.
4. The average person, rolling a 50 with 1 round of stun will only have 30% activity.