Ok, the first thing to bear in mind is that Server 2008 R2 is 64 Bit only, so if your currently running the 32 Bit version of Server 2008 then an in-place upgrade is a no-go, you'll have to do a fresh install
You also cannot do cross SKU or cross languge upgrades.
With that in mind, an upgrade from 2008 x64 was a relatively painless experience for me. Admittedly this server was just a standard file and print sever, with nothing special on there, but it was a smooth experience, everything worked afterwards and had no problems since.