Strengths:

Well-documented efficacy in acute mania and maintenance therapy, but less so for bipolar depression (OFC)

• Following remission of an initial episode of mania, patients should be withdrawn from their benzodiazepine or antipsychotic once the acute episode has resolved. [Australian Guidelines]
• Olanzapine prevents relapse of mania but is relatively less effective against depression in long-term use. [BAP Guidelines]
• Evidence is available for the efficacy and safety of olanzapine in recently manic patients for 1 year, in recently depressed patients for 6 months, and OFC for 6 months. [ICG Guidelines]

Well-established atypical antipsychotic drug

• For patients who continue to have depressive symptoms and are psychotic, clinicians should consider the additional use of olanzapine or OFC. [ICG Guidelines]

Weaknesses:

Causes weight gain

• Valproate, lithium, and several antipsychotics all cause significant weight gain in long-term treatment. Weight gain is associated with other adverse metabolic effects (e.g. carbohydrate intolerance/type II diabetes and hyperlipidemia) and hypertension. [WFSBP Guidelines]
• In short-term placebo-controlled trials, somnolence was the most common side effect associated with olanzapine. Other common side effects included constipation, dry mouth, increased appetite, and weight gain. In long-term studies, 56% of olanzapine-treated patients gained >7% of their baseline weight. In retrospective analyses of patients followed for a median of 2.54 years, the mean and median weight gains were 6.26kg and 5.9kg, respectively. [APA Guidelines]

GSK counterclaims:

Olanzapine has never been studied as a maintenance treatment in patients who were initially depressed at study entry

• The available medicines (for long-term treatment) are probably more effective against one pole than the other. Lithium monotherapy is probably effective against both manic and depressive relapse, although it is more effective in preventing mania. Olanzapine prevents manic relapse more than depressive relapse. Lamotrigine prevents depressive relapse more than manic relapse. [BAP Guidelines] 
• Olanzapine prevents the relapse of mania but is relatively less effective against depression in long-term use. [BAP Guidelines]
• Evidence from double-blind trials is not yet available for the effectiveness of OFC as maintenance treatment for bipolar I depression. [ICG Guidelines]

Olanzapine has never been studied as a maintenance treatment in patients who were initially depressed at study entry. Furthermore, it was shown to be no better than lithium in preventing depressive episodes

• Olanzapine prevents the relapse of mania but it is relatively less effective against depression in long-term use. [BAP Guidelines]
• The available medicines (for long-term treatment) are probably more effective against one pole than the other. Lithium monotherapy is probably effective against both manic and depressive relapse, although it is more effective in preventing mania. Olanzapine prevents manic relapse more than depressive relapse. Lamotrigine prevents depressive relapse more than manic relapse. [BAP Guidelines] 

Olanzapine was shown to be no better than lithium in preventing depressive episodes

• Olanzapine prevents relapse of mania but is relatively less effective against depression in long-term use. [BAP Guidelines]
• Olanzapine and placebo reduced depressive symptoms at a similar rate in patients with a history of rapid cycling. [ICG Guidelines]
• The available medicines (for long-term treatment) are probably more effective against one pole than the other. Lithium monotherapy is probably effective against both manic and depressive relapse, although it is more effective in preventing mania. Olanzapine prevents manic relapse more than depressive relapse. Lamotrigine prevents depressive relapse more than manic relapse. [BAP Guidelines] 

Weight gain has been a significant issue for olanzapine treatment in every clinical trial presented to date. Depression and somnolence are consistently among the most frequently observed side effects with olanzapine maintenance treatment

• In short-term placebo-controlled trials, somnolence was the most common side effect associated with olanzapine. Other common side effects included constipation, dry mouth, increased appetite, and weight gain. In long-term studies, 56% of olanzapine-treated patients gained >7% of their baseline weight. In retrospective analyses of patients followed for a median of 2.54 years, the mean and median weight gains were 6.26kg and 5.9kg, respectively.  [APA Guidelines]
• Valproate, lithium, and several antipsychotics all cause significant weight gain in long-term treatment. Weight gain is associated with other adverse metabolic effects (e.g. carbohydrate intolerance/type II diabetes and hyperlipidemia) and hypertension. [WFSBP Guidelines]