Anyone who's experienced the horrors of upgrading OBIEE 18.104.22.168 to 22.214.171.124 (I documented it here, but even I got it wrong initially and had to update the posting), or has tried to upgrade OBIEE 10g to 11g and hit issues with their catalog, customizations or RPD, will be pleased to note that Oracle have revamped the OBIEE 11g Upgrade Guide along with the 126.96.36.199 release, to include more details on patching between OBIEE 11g versions, and issues you might hit when upgrading a complex 10g installation to 11g.
If you've tried to upgrade OBIEE 188.8.131.52 to 184.108.40.206, one of the issues you'll hit is that the OBIEE-specific upgrade steps are buried amongst the wider Oracle Fusion Middleware Patching Guide, and it's tricky to follow all the links and work out what's needed for OBIEE, and what's not. The new updated upgrade guide includes a section specifically on Moving from 220.127.116.11 to 18.104.22.168, and summarizes the steps that you'll need to perform as part of the in-place upgrade. There's also a note at the end of this section about upgrading BI Apps from 22.214.171.124 to 126.96.36.199 and how 188.8.131.52 supresses many of the RPD warnings that you'd get if you just used 184.108.40.206, so it's worth scanning through this if you're considering a BI Apps upgrade to make use of OBIEE 11g.
The other useful new section in the guide is around upgrades from 10g to 11g. For simple upgrades the Upgrade Assistant that ships with OBIEE 11g does the job well, but customers have been reporting difficulties particularly with heavily customized UIs and web catalog content, and so the Planning to Upgrade from Oracle BI 10g to BI 11g and Oracle Business Intelligence Post-Upgrade Tasks and Considerations sections have both been beefed-up with new content that will help you prepare for the upgrade, and deal with any elements that don't upgrade smoothly.
The new OBIEE 220.127.116.11 Upgrade Guide is available on OTN now, and I'd advise you to take a look if you've only seen the 18.104.22.168 version and found it a bit lacking.