In the introduction to his book "Introducing Buddhist Abhidhamma", U Kyaw Min describes it as a Valley of Dry Bones:
BOOK I, Part 1 - Abhidhamma, INTRODUCTION
Abhidhamma is the 3rd or last Basket of the Buddhist Scriptures. It is said to be abstruse, profound and subtle. It has been described as a Valley of Dry Bones. This Book is an attempt to put some flesh on the dry bones and may be regarded as a Manual introducing Abhidhamma. During the few weeks directly after his Enlightenment, the Buddha intuitively acquired the Abhidhamma and it is therefore about the earliest product of his thought. This is conclusively proved by the internal evidence of the first two Sermons which he preached to his former 5 Companions, called the 5 Vaggi. The first Sermon is called the Discourse setting the Wheel of the Doctrine in motion. The second is the Anatta-Lakkhana Sutta, called the Discourse on the characteristics of anattā. In the first Discourse, he was telling the 5 Vaggi why he can declare that he was the Buddha, the Enlightened. They refused to listen to him at first. The Sermon lasted 5 days but it is very concisely adumbrated into 2½ pages. In this Sermon the Buddha explained that the 5 constituent groups of existence, which are the objects of clinging, are Suffering: this is Abhidhamma, which in this book has been called the 5 Aggregates and Clinging Aggregate. The Second Sermon is purely Abhidhamma, dealing as it does, with corporeality, sensation, perception, kamma-activities and consciousness, and the 11 different distinctions of each Aggregate. However legend has it that it would be necessary to expound the Abhidhamma in one sitting, and as it would take 3 whole months in human time, this was impossible in the human world. It was 7 years after his Enlightenment, during the 3 months of Lent, that he went up to the world of the Devas where his former mother was reborn, and taught the Abhidhamma non stop. Everyday, however, he took time off for his food, and left a Buddha after his own image, conjured up by his miraculous power, to carry on his good work. He also taught his Chief Disciple Sāriputta, who had a marvelous mind. It was Sāriputta who taught the Abhidhamma to his 500 Disciples. Abhidhamma now forms the third Basket of the Buddhist Scriptures, and consists of 7 treatises. The last is the Patthana, also called the Big Book which alone takes up 5 voluminous sections.
The reader must supplement his knowledge of Buddhism by reading the books written in conventional terms. But it is only by a knowledge of the Abhidhamma that even the Discourses of the Buddha, embodied in the Second Basket of the Buddhist Scriptures, can be understood in their full and proper meaning. The ideas about ultimate reality form the back ground of Insight Meditation. Insight Meditation leads to Path Wisdom and to Nirvana, which is our Goal. Everything else is a waste of valuable time. The following is an excerpt from the Expositor 1. p.37: "And tradition has it that those Bhikkhus only who know Abhidhamma are true preachers of Dhamma; the rest, though they speak on the Dhamma, are not preachers thereof. And why? They, in speaking on the Dhamma, confuse the different kinds of Kamma and of its results, the distinction between mind and matter, and the different kinds of states. The students of Abhidhamma do not thus get confused; hence a Bhikkhus who knows Abhidhamma, whether he preaches the Dhamma or not, will be able to answer questions whenever asked. He alone, therefore, is a true preacher of the Dhamma."http://www.scribd.com/doc/489460/Introd ... Abhidhamma