Updating should offer the user 3 possible actions: "Accept", "Decline" and "Preview the update"

[Drugoy]

"Preview the update" option should allow the user to preview the update without actually updating. This will let user decide whether to accept or to decline the update.

[grssam]

May be the user flow could be like this : Only provide two options : Update silently or not.

Notification bar appears telling user that styles have update with 3 buttons : [Preview] [Ignore] [Update]

Preview will apply temporarily, disabling the preview after 1 minute automatically.

When we have per style update, the preview button would take you to the addons manager where you can preview each style separately and apply it or not.

[Drugoy]

Yeah, I think I'll agree: AOM (Add-Ons Manager) already has the options (at the top right corner) where the user may choose between "automatic silent updates" and "automatic updates requiring user confirmation". So now I think having only these 2 states is fine.

As for your idea about 3 buttons - I also agree, but just to make things clear: what will "ignore" button do? Unlike add-ons, styles have no versions, so when there is an update: all you can is to compare the locally installed style's code and the one at the %install_URL% of that style. I'm not sure if Stylish does this, but this is what USM could do: each style posted on userstyles.org has a datestamp of the last update [example: "Updated Apr 22, 2012"], so you could teach USM to grab that info and based on this - you could make "Ignore" to not prompt the user with the updates for that style, if the updates page current datestamp matches the one that it has when the user clicked "Ignore".

[grssam]

When AOM integration is complete, all these options can be put inside the More Info page that will be opened when you click on the Options button of the Style (Just like for addons)

So user could choose there to allow per style, automatic updates on/off/silent.

The ignore would simply ignore that update, and the next time USM checks for updates, it will again show up. This was what I was thinking. But if th estyles have a updated timestamp associated with them, then yes, it can be stored and that update can be permanently ignored.