I have to say that the improvements in the SQL 2008 installation have been pretty good to me. We have a Hyper-V lab with automated SQL installation based on configuration templates which works really well. We can get a box provisioned with SQL 2008 in under 20 minutes. We can do it in a few minutes using sysprep, but that's a no no for supportability and a few other reasons.
Recently I saw a case where the installation did not go so smooth with a cryptic "Access Denied" message which is reported on connect. http://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=390424
and blogged about here
This led me to take some notes on a generic approach to troubleshooting a failed SQL 2008 install and the following steps.
a) Review the "details.txt" file in the setupLog folder C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\Log.
b) Cross reference the failure message in the log file with a search engine, Microsoft Connect and KB articles.
c) If still no resolution, contacting Microsoft Premier Support will probably save time as a next step as they can search internal databases and escalate.
If you want to troubleshoot more these steps have worked for me
a) run "gpresult /V" to output resultant group policy (especially security policy). Compare this to a known working install and see if any core differences. Instructions on gpresult are here http://support.microsoft.com/kb/321709
b) Finally use the sysinternals "Process Monitor" tool to watch the install and identify the resource which is causing issues