Is it safe?
Yes it is very safe!
According to the UK National institute for Clinical Excellence (NICE: http://www.nice.org.uk):
No significant complications were reported in the studies. The most commonly reported adverse events associated with the procedure were abdominal pain, reported adverse events associated with the procedure were abdominal pain, nausea, and vomiting. Delayed passage of the capsule was also reported in a number of studies and in the majority of cases was resolved without incident. In a study of 200 patients done to assess the complications associated with the use of capsule endoscopy, 6 (3%) patients had complications associated with the procedure. This included 1 patient who was unable to swallow the capsule, 1 patient who inadvertently aspirated the capsule and 2 patients who experienced delayed passage and had to have surgery to remove the capsule.
Ref: NICE Interventional Procedures Overview: Wireless capsule endoscopy