An official expansion by another company makes for odd bedfellows.
To begin, Hellfire is only a singe player expansion. This is thoroughly disappointing because Diablo was a great game offline but the game shined in multiplayer. There are hacks and other things but this won't allow players to play Hellfire on Battle.net, Blizzard's free internet support system. The fact that the game is only single player makes this expansion not nearly as tantalizing as the original game and that is probably the biggest knock against it.
Though it might not do well in the multiplayer region, the game does do a few things quite well. Adding another class, the game expands it's ranks to four with the inclusion of the Monk class. The Monk would be easiest to describe as a hybrid Fighter-Mage that is so-so in melee and magic and therefore can not use the top tier items of either the Warrior nor the Sorceror. Though it's neat to play the Monk for a while, I personally prefer the aforementioned Warrior or Sorceror.
Also, the game introduces a few new features such as the ability to jog in town to cut down on the time consuming and cumbersome walking pace from the game before. You can toggle the jog on and off but why would you? I ran all the time in town and it was a welcome addition.
Story-wise, the game isn't going to surprise anyone too much as one of Diablo's underlings, Na'Krul, has now rebelled against his master and this pits up up against eight new levels of chaos from the cyrpt to the insect hive. The levels are separate from the original 16 floors of doom but they are integrated fairly well into the game so it doesn't seem too out of place.
Each zone is four levels, just like the previous game, with all new monsters, shrines, and items. The graphics are the game are just like the original Diablo. The monsters each have a unique way of dying and it runs smoothly without much of hitch. There are new bats and bugs for players to slice through and new barrels, or bug pods, that explode with gunk going everywhere.
As for the sounds of the game, Sierra did a competent job of adding new voices to the Monk class and all the other monsters especially when they die. I can not say they are nearly as good as the Doom Knights biting the dust but they are still good in their own right. None of the sounds seem out of place and you could swear that maybe Blizzard made this game after all (but no multiplayer.... ARGH!)
The game does allow you to collect new items and spells such as Nova and Apocalypse that were only allowed as scrolls or staves before anywhere in the game so you do not have to go to the new areas solely to get new items. There are also a few dozen new magical attributes that can be randomly generated onto items so that also spices things up a bit with a handful of new unique items. My favorite was the mace that shoots charged bolt. Though it's fairly useless, it is neat looking!
In the end, Hellfire provides a suitable experience but only for those that are craving an addition to the original Diablo game. As was mentioned, there is no internet play and that is the greatest knock against this game. Because of that reason, this game is impossible to recommend except to those that want to play single player with only a handful of new content.