I first caught a slight hint of differences within Dispensationalism many years ago when I discovered that there was a preference for the "Old Scofield Reference Bible"(1917), over the "New Scofield Reference Bible"(1967). In those days I didn't care that much to pursue the matter. I only knew there were two camps on the Scofield Bible.
After I "became Reformed" (or, on my way at any rate) Jerry DeHaven and I ran across a book which we found to be devastating to dispensationalism (or, so I thought at the time). And, that book was a first edition (early 90s) of "Wrongly Dividing the Word of Truth: A Critique of Dispensationalism", by John H. Gerstner (R.C. Sproul's theological mentor).
I thought Gerstner's book was devastating, until I read a critique/rebuttal by the then Vice-President of "The Master's Seminary", Dr. Richard L. Mayhue entitled: "WHO IS WRONG? A REVIEW OF JOHN GERSTNER'S WRONGLY DIVIDING THE WORD OF TRUTH".
Later, it turns out that it was admitted that Gerstner's book contained a number of research errors, and the publisher allowed it to go out of print --and that publisher went bankrupt anyway. [After Gerstner's death, "Soli Deo Gloria Ministries" obtained the rights to the book and came out with a 2nd edition which was supposed be a correction of the earlier problems -- edited, I think, by Sproul.] And, now I see at Amazon there is a 3rd edition. Beyond the research errors, the furor seemed to suggest to me that the Reformed writers could engage in "straw men" just like anyone else.
I came away sensing that dispensationalism was flawed, but I had doubts about just how flawed. If "Reformed" writers were going to engage in fallacious reasoning, straw men, and sloppy research then was there no other way to discover accurate differences than to do original research oneself? I thought this might be an isolated example, but further along in the process I ran into Curtis I Crenshaw's diatribe " Lordship salvation: The only kind there is : an evaluation of Jody Dillow's The reign of servant kings and other antinomian arguments", which mixes a critique of dispensationalism and a critique of "easy believeism" in one long hyperbolic harangue.
Then, as a refreshing relief I ran across Westminster Seminary professor Vern Sheridan Poythress'"Understanding Dispensationalists". In it he notes a couple of different problems. (1) The absence of a genuine effort to understand the other side when producing polemics. And, (2) the dated-ness of critiques produced by some Covenant theologians due to the fact that oft' times in their research (as in the case of Gerstner) anti_dispensationalists use older material which doesn't reflect the current thought in dispensational scholarship.
In the case of the former, a dialogue group was established whereby periodically professional theologians from both sides could get together to discuss differences. And, Poythress in the case of the latter [loosely paraphrasing] admonishes his fellow Reformed theologians to keep current with the latest in dispensational theological work.
Dispensationalism is kind of a moving target. The descriptors for the different phases of its evolution (which I am familiar with) are thus:
1) "Classic Dispensationalism" approx. 1830 'til about 1960, represented in the writings of Darby, Scofield, H.A. Ironside, Lewis Sperry Chafer, and a number of others -- basically all dispensationalists 'til the mid 20th century .
2) "Modified Dispensationalism", approx. 1960 'til the 1980s represented in the writings of Charles Ryrie and the younger theologians of the period, including the editors of the "NEW Scofield Reference Bible ". The Modifieds (as I understand it) tried to clean up some of the more speculative and difficult to defend aspects of early dispensationalism.
3) "Progressive Dispensationalism", approx. 1980s 'til the present. Progressive Dispensationalism (as I understand the development) grew (or, is growing still) out of two impulses:
(A) the continued modifying work of the systematic theology of dispensationalism by insights from exegetical scholarship, and from biblical theology. And, (B) it is being modified because of apologetical concerns raised in discussions/debates with non_dispensational theologies (principally Covenant Theology).
Besides Poythress' book, some modern dispensational works which, in various ways, will help the reader understand where dispensationalism has gone (or, has come to) in the last twenty years are: "Five Views on Law and Gospel, various authors, "The Case for Progressive Dispensationalism, by Robert L. Saucy,"Progressive Dispensationalism, Darrell L. Bock and Craig A. Blaising.