I don't like it, Carruthers. It's just too quiet.
Well, I've done the pre-production server, the main live server and the Reporting/BI server with remarkably little trouble. Pre-production and live were rebuilds. I failed live over to our log shipping standby for the duration, which has a gotcha I blogged about before. When I failed back to the primary live server again, it was very quick to bring the databases online. I understand the databases don't actually get upgraded until you recover them but there was no noticable delay. It's gone from 2005 Workgroup - limited to 4GB of memory - to 2008 R2 Standard so it can now use nearly all of the 30GB in the server. It's soo much faster.
The reporting/BI server I upgraded in situ. This took a while but, again, went smoothly. Just watch out, because the master database was left at compatibility level 90. Also the upgrade decided to use the reporting service's credentials for database access when running reports. It didn't preserve the existing credentials and I had to go into the Reporting Configuration Manager to put them back in. Make sure you know what credentials your server is using before you upgrade.
All things considered, a fairly painless experience. Now I just have to upgrade and reset our log shipping standby server again!