The effects list on erowid should tell you all you need to know about datura.

POSITIVE

* euphoria
* surreal interactions with world

NEUTRAL

* hallucinations, elaborate interaction with hallucinations
* smoking of phantom cigarettes
* expands bronchial passages (sometimes used to treat asthma)
* loss of appetite, thirst
* inhibits digestion
* clipped, flat speech
* slurred speech
* loss of sense of the passage of time

NEGATIVE

* dizziness, headache
* dry mouth, dry eyes
* inability to focus eyes (can persist for multiple days)
* nausea, usually without vomiting
* feeling of weakness
* sedation, stupor
* uncoordinated movement
* fear, anxiety, panic
* confusion, delirium, impaired cognitive function (sometimes leading to extremely dangerous behavior)
* amnesia
* blockade of sweat glands, impairing heat dispersal
* difficulty breathing
* numbness
* cardiac failure possible at large doses

When you bring up the dosing information on erowid for datura, you will get the following message.

As a general rule, Erowid neither recommends nor discourages the use of any psychoactive, however, in the case of tropane alkaloid-containing Solanaceae plants, we think it is important to note that an overwhelming majority of those who describe to us their use of Datura (and to a lesser extent, Belladonna, Brugmansia and Brunfelsia) find their experiences extremely mentally and physically unpleasant and not infrequently physically dangerous.

There is no way to guess a reasonable dose, because potency of the plant material itself and appropriate dose for an individual appear to vary so much. The question of what "reasonable" means for something that so often leads to such ridiculously negative effects is hard to say.

As an admixture plant in some traditional ayahuasca brews, smoking blends, or other preparations, parts of these plants are generally used in very low doses. Because of the serious adverse consequences associated with tropane alkaloids (scopolamine, atropine, hyoscyamine) intoxication and the wide variation in doses used, we are unwilling to even speculate about specific dosages for the leaves, flowers, and seeds of these plants.

If you are considering ingesting Datura-group plants, please read extensively from the collected experience reports and never take them without a reliable, sober sitter who understands the likely health issues and who can stop you from walking in front of traffic. Please note that fatalities from ingesting Datura do occur and hospitalizations seem to be quite common, in spite of the relatively low rates of psychoactive use of these plants.