The ideal world:
"It is highly recommended that you perform a migration upgrade by installing SharePoint 2010 and then migrating your SharePoint content and configuration settings from your existing version to avoid problems that can be caused during an in-place upgrade or anytime after the in-place upgrade"Microsoft SharePoint 2010 Administrator's Companion chapter 22 "Upgrading to SharePoint 2010"
The real world:
My manager handed me a migration document in which he chose to go ahead with "in-place upgrade" even though he mentioned that "This approach is risky as there is no easy back out plan. All of the break-fix work would have to be accomplished during a single outage window and users would not be able to access the farm during the upgrade process."
I can just speculate that the decision was made based on the time, resource and budget constraints. It's worth to notice that the migration plan also includes the migration upgrade for another farm, which should merge with the first one.
I have decided to start an online project where I can update online community with real world challenges that I am going to meet during the upgrading SharePoint 2007. The project starts July 1st and is supposed to end in December 2011.
Here is a brief description of the SharePoint 2007 "real world" farm that we are going to upgrade to 2010:
2 Web servers (load balancing) 64x
1 Index server 64x
2 SQL servers 64x
1 Shared Service site
1 Web application with 2 sites collection (the overall size is 15 Gb) with around 40 subsites
Another web farm with no knowledge at this time regarding size and infrastructure, which should merge into the upgraded 2010 farm.
Around 5 sites are created from unique custom site templates. The custom solution are shipped as wsp solution files and includes different types of feature as such: module (aspx files, files with deployable SPD web parts, application pages), event receivers, timer job, SPD and code-base workflow, custom controls, list template,HTTP handlers.
There are 10 BDC application that are in use. BDC apps use the Web service connection to BizTalk.
There 2000 users are created in User Information List. The active users I would say - 100.
SSRS is in integrated mode with SharePoint. The reports are build based on OLAP, data comes from SharePoint to SQL through a custom ETL component.
Summary: There are sites that are highly customized which produces upgrade challenges and for sure will create lots of insights to share with you, guys!