First a general point: dhiker goes with other requirements for success, such as a healthy body and mind that in turn correlate with many other things such as sound life choices, avoidance of sins, having good relations, and a spiritual connection. Dhikr can't be a substitute for these other important things in life and religion. It is certainly not one definite path to "dramatic improvement in all spheres of life!"
Here is a relevant saying from Imam Ali ibn Musa, ar-Ridha, a pious scholar from the bloodline of the Holy Prophet (pbuh) on the importance of a balanced life for a sound religious experience:
اجْتَهِدُوا أَنْ يَكُونَ زَمَانُكُمْ أَرْبَعَ سَاعَاتٍ سَاعَةً مِنْهُ لِمُنَاجَاتِهِ وَ سَاعَةً لِأَمْرِ الْمَعَاشِ وَ سَاعَةً لِمُعَاشَرَةِ الْإِخْوَانِ الثِّقَاتِ وَ الَّذِينَ يُعَرِّفُونَكُمْ عُيُوبَكُمْ وَ يَخْلُصُونَ لَكُمْ فِي الْبَاطِنِ وَ سَاعَةً تَخْلُونَ فِيهَا لِلَذَّاتِكُمْ وَ بِهَذِهِ السَّاعَةِ تَقْدِرُونَ عَلَى الثَّلَاثِ السَّاعَاتِ لَا تُحَدِّثُوا أَنْفُسَكُمْ بِالْفَقْرِ وَ لَا بِطُولِ الْعُمُرِ فَإِنَّهُ مَنْ حَدَّثَ نَفْسَهُ بِالْفَقْرِ بَخِلَ وَ مَنْ حَدَّثَهَا بِطُولِ الْعُمُرِ حَرَصَ اجْعَلُوا لِأَنْفُسِكُمْ حَظّاً مِنَ الدُّنْيَا بِإِعْطَائِهَا مَا تَشْتَهِي مِنَ الْحَلَالِ وَ مَا لَمْ يَثلِمُ الْمُرُوءَةَ وَ لَا سَرَفَ فِيهِ وَ اسْتَعِينُوا بِذَلِكَ عَلَى أُمُورِ الدِّينِ فَإِنَّهُ نَرْوِي لَيْسَ مِنَّا مَنْ تَرَكَ دُنْيَاهُ لِدِينِهِ وَ دِينَهُ لِدُنْيَاه
Struggle to divide your time in four: a time for prayer, a time for earning livelihood, a time for meeting trustworthy brethren and those who inform you about your faults but love you sincerely in their hearts, and a time spared for your pleasures (or leisure). And by this last time you will be successful in the other three times... Allow your self an enjoyment of the world from what you desire from the lawful enjoyments and that which doesn't violate perfection and doesn't exceed. And use these enjoyments to aid yourself in fulfilling matters of religion as it has been narrated (from the Messenger of Allah pbuh) that he who abandons the world for religion or abandons religion for the world is not from us.
In my experience too many people who become fretful with their spiritual life must check their lives for imbalance in the four occupations explained in the above narration.
An imbalanced lifestyle can led to an imbalanced mental state, impairing our ability to a maintain a fruitful spiritual life. Mental health is generally key not just to worldly success but spiritual progress for mind is the ruler of our existence.
Moreover, in the tradition of Irfan and spirituality, not all forms of dhikr are recommendable to the lay person. Shia spiritual scholars particularly discourage lengthy utterances of tahlil for it is the most overwhelming Islamic doctrine and its full significance can't be born by the average person. They say this dhikr invokes the highest level of Divine Awe and Majesty which can be psychologically overwhelming to the uninitiated. They in fact cite individual clerics who permanently impaired their spiritual progress by excessive concentration on the wrong dhikr causing a negative feedback in their soul towards spirituality in general!
For they laymen, they instead recommend dhikrs that have gentler meanings such as Alhamd u li Allah, Ya Rahman o Ya Raheem, Ya Latif, or salutations upon the Prophet (pbuh). And they may discourage long dhikr at all and rather advise the person to suffice to the obligatory ibadat such as the five time daily salat.
At any rate, these religious mantra more than doing any miracle, will help engender a better attitude to God and spirituality in one's heart, and that must be the primary goal.
Note that in the above discussion, by dhikr, repeated recital of specific phrases was meant, not general remembrance of Allah that is a recommended state for all believers.