Doom 3: Resurrection of Evil is an expansion to Doom 3 and costs only $29.99. It takes place 2 years after Doom 3 and with a large similarity to it (which is why it isn't $50). And of course you get to battle demons without much help from anyone else. The game has a few differences from Doom 3 but is there enough to warrant even $29.99?
The main problem that people had with Doom 3 was that you had to switch between flashlight and gun. You couldn't attach your flashlight to your gun and this became a bit annoying rather than adding to the scariness. So now you have a light attached to your pistol. Except it is only on your pistol so it is only a partial solution. It seems they did that so you weren't as helpless but still to switch to a powerful gun while in the dark. Speaking of guns, there are also some new additions to available weapons. The best being the powerful double-barrel shotgun. To compensate for the power, reloading is slow. The other new gun is "the grabber". It allows you to pick up some the objects in the environment. Think of the Psi-Ops power but controlled through a gun instead of mentally. The nice thing about the "grabber" is that it doesn't use any ammo or energy of any sorts to use it, but you can only hold onto thing for a short period of time. The best time with the "grabber" is when you catch blue fireballs thrown at you and throw them back at the demon throwing them at you.
Really the main change in Resurrection of Evil is the focus of the game. At the start of the game you find an artifact that is capable of hell powers. It will allow you to go into 'Hell Time' aka bullet time. As you go through the game, it will get more powerful and give you more ability.
In a turn of events, they not only added these things but took out something that will disappoint people. Co-op mode is no longer available to be played online. There is still online deathmatches of course, considering it's mostly the same as Doom 3. Capture the Flag is also missing in this version. And those that bought the limited edition Doom 3 for the classic doom ports will be disappointed to know that they are now included with Resurrection of Evil.
Resurrection of Evil is a short game that is very similar to Doom 3 (hence expansion) but it is only $30. The answer to the question on whether the game is worth a purchase, rental, or neither is going to depend on how you enjoyed Doom 3. If you can't wait for the next Doom title, this will probably help satisfy your cravings for a little longer. If you had some fun with Doom 3 but haven't played it much or thought about it in a while and aren't die-hard, Resurrection of Evil may not have enough to warrant even the budget $30.