A Charm spell has an immediate effect: it turns the subject into a
friendly attitude (if not already or better), which means (from SRD) "Wishes you well: chat, advise, offer limited help, advocate". It does not turn the target
helpful ("Will take risks to help you: protect, back up, heal, aid"). I have to say that IMHO it doesn't mach very well with what the same spell says about treating you as a trusted friend and ally, which in my opinion it just means helpful, not only friendly as the technical definition...
Then the spell lets you give the target orders, to do which you are required a Charisma check only if the order is something he would not ordinarily do (attacking a former ally?), and harmful/suicidal orders are never successful.
To play it simply, I think the DM should play the target as the definition of friendly:
1) out of combat, he can help as much as he can (if asked to do something with possibly dire consequences to himself*, do the Cha check with an appropriate circumstance modifier depending on how risky the request)
2) in combat, he doesn't try to harm the caster in any way, but doesn't automatically provide beneficial spells or attacks the caster's attackers (that would be helpful); he may do such things only if the caster tries a Cha check to convince him (DM must adjudicate the circumstance modifier, for ex. penalty to attack his former allies, but not so big penalty to attack the "cannon fodder" goblins)
*non directly harmful, such as a charmed guard letting the PC enter the castle when it would cost the guard to at least lose his job
It's very complicated to run such spell effects anyway, the description even says that the target "percieves your words and actions in the most favorable way", which could imply that if he sees the caster fighting the goblins, he may decide that it's the right thing to do himself...
Let's remember that it's only a 4th level spell, and it has a very long duration (days), it is quite too good if it provides a full functional ally
...and yes it has a save (and +5 if used in combat).
edit: Forgot to say why I have said this... If we treat a charmed monster as an ally, it's too good; if we treat it as just non-hostile for the caster, it is perhaps too limited. Maybe the key of the whole thing lies in the Cha checks, which are probably going to be some of them successful and some others not. The DM has anyway a tough job in adjudicating solid +/- to different orders.
