In secondary prevention aspirin has a well proven role, with a 0.5% reduction in the risk of stroke. The Antithrombotic Trialists' Collaboration found that the absolute reduction in the risk of having a serious vascular event was 36 per 1000 patients with previous myocardial infarction treated for two years. This study was published in the British Medical Journal in 2002. You can find the article in Volume 324 on page 71.
The benefits of aspirin therapy in primary prevention are not as clear cut. It may be that it is a reasonable to treat men who are at high risk of vascular events, but the balance of risks and benefits is less clear for people at lower risk, and for women.