It would be a migration not an upgrade, there has never been an upgrade path for any version of SBS. It depends I guess on whether you think the benefits of SBS 2011 over SBS 2008 are compelling enough and or your hardware is due to be updated. Personally I wouldn't consider the differences between Windows 2008 and R2 and Exchange 2007 and 2010 enough for me to migrate if I just setup an SBS 2007 server in the past 18 months.