When I think of Risk, the board game, I think of sitting around a table with a bunch of buddies, arguing loudly, cursing dice-rolls, and just generally being aggravated for several hours.
For one thing, it gives you the opportunity to play solo, something you can't do with the board game. You can declare war on two-five AI opponents and choose between three difficulty levels.
On the multiplayer side, Risk lets you "pass and play" with up to six human players (adding computer players into the mix as desired), or go the multiplayer route--but only locally via Bluetooth or Wi-Fi. Obviously, an online option would be great, but given the time involved to complete a game, I can understand why EA elected to omit it.
This Risk plays just like the board version, but with a few obvious changes. First, you can distribute your armies manually or save time by letting the app do it for you (it's reasonably proficient at strategic placement).
Second, there are no physical dice to roll. When the time comes to attack, you tap an icon for an individual roll (or "skirmish"). There's also a "total war" option that essentially plays the entire attack until there's a victor, no rolls required. This makes the game go considerably faster, though it does leave you without a retreat option (which is available if you skirmish).
The gameplay mechanics are a breeze to learn, and the app includes a tutorial for newcomers. I did discover a couple quirks, a big one being that there's no way (that I can find) to see what or how many cards you've accumulated. Without that information, planning your strategy is a lot more difficult.
Even so, I had a blast playing Risk on my iPhone (and my iPad, where it looks reasonably nice for a non-optimized game). It's nice to be able to zip through a game in 10-20 minutes instead of 2-3 hours. Definitely worth $4.99 if you're a fan.
Speaking of classic board games for the iPhone: