Since your Guru is the root to all your attainments, reciting your guru's mantra is just as powerful. Without your guru, you would not meet the Buddha, the Dharma or the Sangha. You wouldn't be able to practise any yidams, or might not even hear the Dharma. Therefore with a clean Samaya alone with one's guru, one can gain siddhis and attainments, ward of negative energies, seek of protection, invoke efficacious help.
Thanks for the article, it was very knowledgable. Anyways, too bad for me I can't find a guru over where I'm at..so whenever I'm feeling uncomfortable or just need a little blessing, I recite Dorje Shugden's mantra. It always makes me feel alright instantly.