Alot has been written about Windows Search and how it works, or not, with Enterprise Vault. The main issue comes about because you can get into a mixed bitness configuration. By that I mean Windows 7 64 bit, with Office 2010 32 bit. This causes problems.
Outlook Instant Search - What does it search?
Outlook Instant Search only ever searches the Enterprise Vault Virtual Vault Metadata cache. This is on top of Outlook items. The Enterprise Vault metadata that can be searched using Instant Search is things like the date, author, recipients, subject, and up to 120 characters of the message body. This therefore excludes things like the full body content, and excludes attachments.
How can I search full content?
What about that mixed bitness?
When the Virtual Vault feature was first introduced there were some issues relating to mixed bit-ness. Mixed bit-ness maens that you have 64 Bit Windows, but 32 Bit Office. In some situations this is the recommended configuration from Microsoft, and so is quite common, probably more so than you would initially imagine.
From EV 9.0.3 the issue with searching and mixed bitness has been resolved, and now you can search quite easily, regardless of the Office versus Windows bitness.
What about on Windows 8?
The ability to search emails has gone from Windows 8. Microsoft did studies and discovered that very few people searched for mails from the 'start' interface on Windows 7 and earlier, so it got chopped out. The net result of this is that for those users who are going to use Windows 8, then the desktop policy shouldn't be set to force WDS on. Otherwise you'll be added work to the end-user workstations with no point - no interface exists to get that data retrieved and displayed to users. So no point in building the index - if it's even possible.
With Windows 8 then, if you're using something like Office 2010 or Office 2013 you'll have the Outlook Instant Search feature.
Personally I think the Instant Search approach is a really good way, provided users are educated to the caveats that exist around what will be searchable. The biggest advantage is that the searching is taking place right there in Outlook, not down at the 'Start' menu area. It's far more seamless. With Windows 8 you don't have the option of searching mail from the 'start' menu like you did in Windows 7, which makes Outlook Instant Search even more likely to the route that users take.