CM is nontoxic, even in large amounts. Creatines effects on blood pressure are an open question. Since it has the effect of fluid retention in muscle, it might increase blood pressure in the same way high sodium levels do, but this has not been established or refuted. In addition to this, one other symptom reported is stomach cramps. Reducing the intake of creatine can lead to a reduction in severity of the cramps.
Creatine seems to be well studied in scientific research.
Scientific evidence supporting creatine is there, but while some very good results have been reported, like a 20 lbs body weight gain in 6 weeks and strength increases, others have reported no significant gains whatsoever while taking the supplement. Like all supplements, supplementing creatine is useless if your body already has enough of it. Further supplementation is then not needed and just a waste of money. If however, you do not have the optimal levels of creatine in your muscle cells, then supplementation is a good idea which can really enhance your training. Some people get minimal or no effect from creatine.