Some of the metamagic rods, in actual play, won't have the same effect that Sudden feats do. They usually won't allow a caster to Empower a spell that he otherwise couldn't Empower -- except for the Greater rods.
Assume that the DMG wealth guidelines are being followed and assume that a caster has a rod worth no more than 1/4 of his total item value. (I'm trying to reflect what a real character might look like.)
The Lesser Rod is then available at level 9 -- so the caster can empower spells of levels 1-3, just as he could with Empower Spell.
The Rod of Empower is available at level 14. The caster gets the ability to empower 6th-level spells one level before he would otherwise.
The Greater rods really throw things out of whack, because they all let you Empower, Maximize, or Quicken spells that you couldn't otherwise. I imagine that a Rod of Greater Quicken is pretty much a must-have item for any archmage.