The fifth edition was when I decided to separate out my collection into battle companies with each company being the corresponding edition. (except of course for the Rogue Trader era Seventh Company). As I came back to the hobby late into fifth edition I had the Assault on Black Reach set along with a couple of other NIB sets and like the fourth edition stuff and these have now been assembled and basecoated. The rhino and land raider I'd had since late third edition but I felt that the fifth company was where they belonged.
Of course, as soon as I had finished dividing my collection the announcement that sixth edition was on its way came......and I had run out of battle companies to use!