I don't doubt there are times when the snail pace SED can resolve some complicated string manipulation easily, however for almost all cases I have found with a little creativity it's more efficient by far to use "Parameter substitution and expansion" already built-in to the shell itself, which is incredibly fast.. I'd recommend mastering this before delving into Sed. Beside, Sed is too slow and consumes a huge amount of resources unnecessarily just to resolve one simple string manipulation.
Check out "Parameter Substitution" under "Advanced Bash-Scripting Guide"
I think Sed is better suited for text manipulation within a document/text file