I've been on Prilosec OTC every day since 2007 (Prevacid before that). I'm still very sick so I'm doing some research and found some interesting information about Prilosec, which may or may not be the same for other PPI's as well.
According to the Prilosec OTC website (http://www.prilosecotc.com/hcp/PrilosecOTCDosage), one pill (20mg) blocks around 60% of stomach acid. But a half pill (10mg) only blocks 20%. Interestingly, higher doses (30mg to 80mg) still block only 60% of the acid, which means that higher doses are not more effective than the regular dose. So why do I think I feel better when I double my dose? Maybe it's the placebo effect? Although when I double my dose, I don't take 2 pills at the same time, I take 1 in the morning, and the second one I'll take later that night. The "duration of action" of one pill is 24 hours.
Also, the pills are enteric coated, so splitting the pill in half reduces its effectiveness because stomach acid breaks down the chemicals so they don't work as well. Because of the coating, the pills only start to work when they reach the less acid environment of the duodenum, which can take a half hour to 3 hours to start working.
The most common adverse effects are headache and diarrhea, but in all the years I've been on it I've never had those side effects.
Hope this is helpful!