Excellent addition (or end?) to the series, innovations are enough to make it feel fresh
First of all, I will confirm that the small touches which make the game feel like a Heroes game have been preserved, or rather have been returned to after seemingly having been abandoned with IV. This is reflected in too many ways than I feel like going into here, but for example spells are not associated with town types, the weird alignment system has been done away with in favor of the old town types, there are two tiers of creatures again rather than having one lair block out another lair's construction, etc.
As for what's new, it seems to me that pacing in the game is faster compared to previous installments, though of course it could just be my perception. I felt as though I could build a main army or two up quickly, send them into battle, and recover faster afterwards.
There are also a huge number of hero abilities and specializations that all have a tangible impact on battle and the game in general. Surprising combinations of abilities unexpectedly unlock other abilities, and hero leveling feels fresh and exciting, at least for now.
The assessment of the campaigns and AI in the main article are pretty much spot on as well. The campaign is engaging enough, and the AI is good, though not perfect. I recall on one map witnessing an enemy army running around in loops rather than engaging me directly.
The graphics and sound are much improved not only in terms of power but also style. In my opinion, IV was definitely a low point in this regard.
If you are a fan of the series, and especially if you have been somewhat disappointed with Heroes IV, I would urge you to give this game a shot. Its price has been going down since it was reviewed on Gamespot, and at $20 it is definitely a bargain for the many enjoyable hours of vanquishing pixies and demons you'll have.