Macros certainly aren't as powerful as they were 5-6 years ago (raise your hand if you remember what you
used to be able to do with a macro...like cast different spells based on class, detect spell resists, etc) and it was the right thing for Blizzard to do to change that, as you really could come really close to having the macros play the game for you.
However, while they have strict limitations, a little creativity and some trial & error can still make them extremely powerful tools. The link that Akkorian posted is my macro-bible and
Fitzcairn's Macro Explain-O-Matic is also a great tool for learning.