FYI: The US Preventive Services Task Force recommends against the routine use of aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) to prevent colorectal cancer in individuals at average risk for colorectal cancer. The main negative effects are gastrointestinal bleeding.
Looking at all-cause mortality (rather than colorectal cancer in isolation) might drastically change the analysis, but I'm not aware of such research having been done. Closest I've come across relates to the Cardiovascular Polypill.
(For Round 1, see this comment from last year.)
NYT: Studies Link Daily Doses of Aspirin to Reduced Risk of Cancer
The article is worth reading in its entirety, but here's an especially interesting paragraph:
The evidence still isn't perfect, but the purpose of rationality is making good decisions with limited information. I am a healthy 28-year-old and these studies make me even more confident that taking daily low-dose aspirin is the right thing for me to do.
On a related note, if society were more rational, I wouldn't have to be sad reading paragraphs like this one:
Or these ones from A Cheap Drug Is Found to Save Bleeding Victims, published on the same day: