how effective are dwarf archers on isengard warriors?
are they worth using or should i replace them with warriors?

They are great against any defence five warriors as their strength three will rip through them on a 5+, but against defence six they aren't as good. But I would still recomend taking archers, even if he has a lot of defence six as they are a huge tactical asset.

I'd take a few, around six maybe, but you won't need many.

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

i would take 8 archers per 500 points in the army
this way you should hit with 4 shots and take out an uruk a turn most times
also if things dont go well they are still strong enough in numbers to get into combat

all in all dwarf archers are worth it

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kill, kill

But dwarf rangers are better.

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"War does not determine who is right, only who is left."
- Bertrand Russel

I prefer a mix of Dwarf Warriors with bow and Dwarf Rangers with bow.

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

Against Uruk-Hai, the rangers are probably better. They have an odd defence value, which is better against the Uruk-Hais' even strength value. Also, their extra range would be valuable. Put some rangers with your regular army as well, and give them throwing weapons. Then, try and get them toward the flanks so they can target the pikes.

But you'd still need 6s against the pikes as they have strength 2. Unless you're talking about the throwing weapons which seems a little off topic.

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"War does not determine who is right, only who is left."
- Bertrand Russel

that is why i would take dwarf archers
isengard armies tend to have a fair few pikes and crossbowmen which have d5 so s3 shots are useful

still using rangers can be a good idea( maybe take a mix of both in a large point game)
all depends on personal preference

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kill, kill