I think HOMM 5, despite some horrendous voice acting and sub standard level designs, is actually a very good game. It was certainly much better than 4 ever was, which I didn’t like particularly (I used to hate having to choose between different level creature types). Most of what they changed was for the best IMO and the addition of racial skills was a fundamentally good idea, if rather patchy in its implementation. I think if you are coming to the series now (or came to it much later than its release), all the patches and stuff have ironed out the bugs and make it a good game.
I think a similar case can be made for 6. I got fed up with the sheer awfulness of Uplay and the constant bugs and crashes and stopped playing it after release. However, since then, I watched a random lets play and thought id give it another shot. I was mildly surprised actually. It runs a lot better now, there are town screens (not in vanilla), the story is ok and the graphics are good. I still think 5 is better, but its enough to keep me playing.
I think the series has pretty much had its day though. I would be somewhat surprised if they made a HOMM7. It needs a breath of fresh air from somewhere IMO. I read an article somewhere that suggested either multiplayer only or IOS was the way to go. I could possibly see it working on the IOS. If you removed nearly all of the story elements and made it pure strategy I think that could prove popular. HOMM3 had next to no story and is the most popular of the series.