It depends. The most likely to be successful case is where the applicant is motivated and competent and the project that the research group can offer matches the applicant's background and interest. Other than that, I think it's not useful at all and it's actually pretty annoying to the professors.
University of Waterloo
个人总结的一些重要的忌讳:
1.别夸自己如何聪明,有创意,成绩好。那是推荐信的事情;
2.别对教授的工作品头论足,泛泛夸对方research等于蜻蜓点水;
3.格式要统一,别弄得花花绿绿,也不用追求美观;
4.别用QQ,sina,163,etc. 这些奇葩的天朝邮箱了!by the way,碰到回复你QQ邮箱的老师,就嫁了吧;
5.别写太长,别说废话