The relevancy of having played previous Heroes games is reduced to zero when considering Heroes 6. The game is just great, whether compared to previous versions or competition.
I'll get through the easy parts fast: visuals are very nice. Smooth animations, good creature artwork all around (even if some will complain about the "excess detail" - from my part that's a question of taste), and amazing map artwork. Many maps start smooth, and then they open in sceneries that just make you stare. From color choice to overall map design, I like it. Everything is easy to see and smooth on the eyes.
Sound is well done. I don't know what you would expect from some creatures, but they seem to make the normal sounds they should. Same for the spells. The music is very well chosen. With inspiration from previous Heroes games, it just makes everything seem well rounded up and nicely connected. Each town has it's own representative theme, with some being quite inspirational. The nice touch is that also different map areas have their own soundtrack, giving for example an eerie feeling while traveling underground or through cemetery-like areas. Voice acting is one huge level above previous games. Even though it's not constant in quality, overall it's good.
Story wise it's a clear improvement from Heroes 5. The story (finally) makes sense and the characters follow their paths truly. There is even character development (outside the rpg gameplay)! So yes, the story is good and definitely worth a walk through.
The gameplay. Ah yes. The many changes. I actually consider them well thought and integrated in the game. The reduction in number of resources makes you focus more on the character progression/rpg part, which has been greatly improved/changed (I'll get to that in a few lines). No more searching/waiting for gems to have your Titans. The town differentiation is even more emphasized that in previous games, with each town having more than one unique building to choose from at various stages of development. This obviously gives a different feel to each faction and game played. The forts and area of control related to towns are a nice addition. It makes for a much more streamlined exploration, as you don't have the constant back and forths from previous games to make sure you keep the mines you need. This is also helped by the fact that you can convert towns to your own faction and buy all creatures in your kingdom from a said town. Again, this is a change that makes it much easier to focus on the battle and rpg part. They might seem like big changes, and they are. But its something that melts in the background and just seems natural after the first map. The rpg part, reflected in the loot and character progression, has been reworked also. Now each hero has a choice in terms of next skill/spell. It's all been put together in a skill tree, equally accessible to might or magic oriented heroes, although the final skills in one category and certain schools are completely accessible only to specific combinations of might/magic heroes and faction. The skill menu itself is a bit overwhelming and requires a bit of getting used to. But the good from this is that the multitude of options and combinations is actually amazing. Besides the fact that it's much easier to create exploration oriented heroes (or other type for that matter), the synergy between some skills is very well thought. This definitely requires time and experimentation.
The bad part in the game: as said by others, there are definitely bugs, but maybe I'm the lucky one as I haven't been plagued by the multitude some people seem to have encountered. Most bugs are visual or sound issues, and I haven't had any crashes or anything seriously similar. The fact that the developers already patched the game for some issues in the first week, and are already working on other improvements (check news/forums/UPlay menu/etc.) makes the bug issue a relative one. With the introduction of the Conflux (which is an interesting incentive to have a constant connection to Ubisoft servers, as it provides extra content while playing campaigns or otherwise) it actually looks like the support for the game is intended to be for quite a while. And judging from the beta and current Heroes, the developers are actually doing a great job at improving the game experience. The Conflux itself I definitely don't see it as a positive. If it weren't for the great game itself, this alone would get one point of the mark. Luckily the developers managed to get some good out of this DRM form. I left out quite a bit, like changes to artifacts, the new faction, but this is worth a look of your own.
All in all the game definitely gets 5 out of 5 from me. It might not have town screens, but that's less bothersome then it seems. Again, a question of taste. For those who love them, just a bit of patience, as it seems to be one of the top priorities for a future major patch. I just hope they put the option of enabling/disabling them. Good sound, visuals, great gameplay and support, and definitely fun. Even if you're not a major fan of strategy, this will eat quite some months of your life. Hopefully more :).