Correct.
They do not change the spell slot. Check.
They also do not change the way the feat is used. Therefore the spell still has to be prepared, only the slot does not increase as per the item description.
Here's one part, which I think is quite important:
Possession of a metamagic rod does not confer the associated feat on the owner, only the ability to use the given feat a specified number of times per day.
This here is often refered to as an argument, why they can be used spontaneouly:
All the rods described here are use-activated (but casting spells in a threatened area still draws an attack of opportunity).
However, since sorcerers can use them spontaneously - as they normally use metamagic - that part has an obvious meaning, it could only refer to spontaneous spellcasters.
And use-activated also applies, if they are
used during preparation.
The specific rod descriptions could also be read, like the spontaneous use was possible:
The wielder can cast up to three spells per day that are empowered as though using the Empower Spell feat.
But regardless of how you apply the metamagic, in both cases you
cast a metamagicked spell, so it works both ways.
Also the following part only really makes sense, if it works the way I say:
A sorcerer still must take a full-round action when using a metamagic rod, just as if using a metamagic feat he possesses.
Since otherwise all spontaneous casters (except sorcerers!) could use the rods without a time increase... but if you use the general rule for spontaneous metamagic (in the same way I say you have to use the general rule for prepared metamagic), then all spontaneous casters have the time increase, and the above quote can be seen as just an example for
the spontaneous caster, the sorcerer.
On top of this comes the balance issues: Why should a sorcerer need to suffer a higher casting time (the "cost" to apply metamagic for spontaneous casters), but a wizard not have to prepare (the "cost" to apply metamagic for prepared casters)... and not even also suffer the time increase then, when even a spellcaster who already
can apply metamagic spontaneously - and therefore obviously can do it "better" - cannot do so without this limitation?
@Pinotage: I think the text was the same, at least it was in no way more specific.
Bye
Thanee