I'm new to the Forum. I actually completed the 3rd edition about one year ago last October. It was an edition I had started 23 years ago in High School and never completed at the time. So, many years later I made up my mind that I was going to finish it once and for all. I'm glad I did.
After finishing Chapter 40, I read throught the Loci Antiquae but then skipped over to Ritchie's and the "Roman History" readings on Dale Grote's site (actually, I bought a hard copy on ABEBooks), just to see how I measured up to "Second Year" college level reading. Then, I went back (with a 6th edition of Wheelock) and read through the Loci Immutati.
For me, the leap from the LA to the LI was quite dramatic, in terms of difficulty. Part of that, I'm sure, is due to being a self-teacher and not always picking things up correctly the first time. Still, I think Wheelock was a good foundation, and I frequently revisit chapters here and there just to reinforce certain concepts and forms. Plus, since the 6th edition has so many more readings per chapter there's a bit of new ground to cover.