I have had nothing but positive experiences with colonics, but have read some semi-horror stories of people who obviously ran into bad therapists/funky equipment.
Personally I have been involved in major colon cleansing for over two years, and could not have made the progress I have without many colonics. My naturopath is my colonic therapist, and knows when to administer one and when it is not advisable, and also makes sure I have the nutritional or herbal support I need to prevent problems such as you describe.
Colonics have truly been a lifesaver for me. I believe it is well worth shopping for the right therapist, with the best equipment available.