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One of the most successful and memorable campaigns I ever ran was structured non-sequentially. The campaign was a (largely) post-apocalyptic "Mad Max"-esque campaign, in which the players bounced back and forth between the pre-collapse 20th century (where they all met for the first time), the "beginning" of the adventure, and a "floating" point of peril in the near future.
I began every sequence with a zinger: a near future point of peril, like them hanging off of an out of control big rig, or some such thing. I made it as exciting and adrenalin packed as possible, so they never stopped to consider how they got in that situation. They would then resolve the fight/conflict/problem in a cool way, usually by the seat of their pants.
When the zinger was resolved, I'd regress things to the beginning of the situation that was just resolved. This would generally be a very role-playing heavy encounter of some sort. The beautiful thing was that the players were always so pumped by what was about to happen that they'd bend over backwards to ensure that the situation came to be... even if they lost in the flash forward, and ended up staked out on the desert highway to die (with their weapons tantilizingly out of reach), they'd have already decided what they want to do when things caught up to the end of the zinger, and thus cooperate with the unfolding story.
When the present had caught up to the beginning of the flash forward, I'd regress things to the flashback by focusing on some element of the situation (usually the description of a physical object like a car, pile of barrels, etc) and juxtapose it with an occurence of a similar situation in the past. I.E. people hiding behind flaming barrels at the end of the present/start of the zinger, would remind them of a similar situation where a combat erupted around stacks of barrels in the pre-collapse past. The flash back sequence always ended as it began, with a juxtapostion of their past situation with an element of the start of a new zinger (flash forward).
The cycle would then repeat: Flash forward, present to start of flash forward, past digression, new flash forward, etc.
In my opinion the adventure worked because of a few things: (1) the characters were new, so we were free to create their pasts as we played, (2) they never directly met or interacted with the past/future villain... flash backs were mostly a matter of information gathering, learning about the capabilities/psychology of their future nemesis, (3) the flash back ended with the onset of the apocalypse, so it didn't seem illogical that their past endeavours remained unresolved (they had bigger things to be concerned with: like basic survival), (4) I rigidly observed the cycle of flash forward/present/flash back, so the player always knew when/where they were in the story (and when/where they were going), and most importantly (5) I had very enthusiastic players.