I'd just like to share that it's a bit easier to read the abhidhamma books in conjunction with their respective commentaries - both the dhammasangani commentary (Atthasalini - Expositor) and vibhanga commentary (Sammohavinodani - Dispeller of delusion) are available in English from PTS. With the commentary to other 5 books (Pañcappakarana), it's a bit harder, but there are bits and pieces of it translated by different authors (notably U Narada). If you happen to live in a big city, usually the biggest library will have some or all the books from PTS (excluding Yamaka as mentioned), so it's not that hard to find them. And of course there's more and more info online (I already mentioned Chew's blog - there are English translations of several sections of Yamaka there now, aside from the lectures and other interesting stuff).
In terms of the relevance of abhidhamma, I remember one of RobertK's posts where he says something to the effect that all the modern vipassana techniques are basically abhidhamma rehashed. I really liked that, because the more I learn about abdhidhamma the more that seems true. Actually, there's a commentarial explanation that says that abhidhamma is most useful for establishing the right view - and this seems very true, especially in terms of understanding the three marks and conditionality (I can't remember now what the other two pitakas are most useful for - I think suttas were for samadhi and vinaya for sila, though of course, each pitaka addresses all these issues in its own way).