I've retained my history of chatting since my first iPhone, the 3GS, until now, iPhone 5. Now my messages have accumulated to over 400mb lol.

I use a iPhone file management software called iExplorer on my Mac (you can use iFun-Box if you're a Windows user), navigate to WhatsApp and copy these two folders:
Documents
Library

Backup these two folders to somewhere on your computer. Update, move to a new phone or whatever you want to do with WhatsApp. After installing a new version or a fresh installation, overwrite the two folders on your phone from what you have backed up.

Here are the links to the software that I use:
http://www.i-funbox.com/ (Windows and Mac)
http://www.macroplant.com/iexplorer/ (Mac)

There used to be a time when the only way to access the file system was to SSH. But these apps are way easier.

That is a viable solution, but most people would recommend that you don't restore from an older phone. With a restore from backup, problems such as battery draining can happen on the new phone. That's why I always try to set up as a new phone. In order to save my application data, I have to manually copy the Document and Library folders from every app lol.

No it is different. The iCloud backup and WhatsApp iCloud backup are different. They occupy their independent space on iCloud.

Let's say for instance you want to restore your WhatsApp data but the only backup you have is the iCloud backup. You'd have to restore your entire phone using iCloud just to get your WhatsApp backup back.

However, with the new WhatsApp iCloud Backup, the backup is stored independently on iCloud and it has nothing to do with the iCloud backup. If you want to restore, just go into WhatsApp and activate it. There's no need for you to restore your entire phone.

Yep, it feels very smooth.

On a sidenote, does anyone know why WhatsApp is constantly stuck at 0% backing up to iCloud?

This issue used to exist because of iOS's nature. No apps can run in the background (even to receive messages). What actually happens is that the server pushes the message to your phone and you receive Push Notifications. These are just notifications, not messages. When you launch WhatsApp, WhatsApp needs to connect to the servers to download the messages.

Well, I wrote "used to exist" because iOS 7 fixes all that by introducing a nifty little feature called Background App Refresh. If you're on iOS 7, go to Settings -> General -> Background App Refresh. Turn it on. Scroll down till you see WhatsApp and turn it on as well. Once that is done, any Push Notifications you receive will trigger WhatsApp to run in the background to receive the messages. Well, of course, it takes a little toll on your battery.

That's interesting. Whenever I launch WhatsApp, with Background App Refresh enabled, I see the messages instantly

I experimented again after what you mentioned. Note: You have to leave the app running. Don't swipe it up in Multitasking. You have to leave it in the bar. Don't force quit.

Either that (disabling Background App Refresh) or force-quitting the app.

No problem mate. I was curious since it did not work for you, so I tried with the app closed. Turns out messages weren't received when it's not "running" in the background. iOS does things differently than other handsets. Even with "full" (Apple calls it full) multitasking built into iOS 7, it still isn't real multitasking. Apps are still frozen when closed, and they're only allowed to run very limited background processes in order not to put a strain on the battery. New to iOS 7 is the ability to run background processes whenever a push notification is received, and that's how WhatsApp receives messages in the background.