Virtually no negative sides. A few reports of increased oiliness of skin, a couple reports of minor headaches, a couple reports of trouble sleeping, and a couple reports of increased heart rate. All of which are norms for supplements that do any of the things RPM does. Most of these side effects can come from any stimulant actually.
Let me know if you have any further questions. Did you get a chance to read the writeup? I strongly suggest you research ANY product you are considering using.
Summary: Overall, I'd guess 95% of users report no negative side effects at all. The few who do are very minor and correctable.