So, you may be a bit confused about how this is a new feature, as pigs have been possible to ride for ages now. Well, finally there's a way to control them, instead of having them run around like mad, normally ending in death...

How do I control them?

Well, you need two things. Firstly a saddle (as always, you can get these from dungeons, and now it's possible to get them as a trade from villagers) and secondly, a carrot on a stick - a new 1.4 item.

There are two things you need to make a carrot on a stick... and no, it's not a carrot and a stick. You'll need a carrot (obviously), and a fishing rod.

Carrots are a new crop, and there's two ways to get it - from the farms in NPC villages, or as a very rare drop from zombies. Unless you want to take rather a long time finding a village, the zombie method will probably end up being the quickest way to get one though.

Then, you just craft them like this:

Is there any real benefit to riding pigs?

I suppose it looks quite good, if that counts as a benefit. However, they are faster than walking, so if you need to get somewhere quickly and don't feel like setting up a minecart track, then pigs are probably the best way.

What if I don't use a carrot on a stick?

Then the pigs will be exactly as they used to be - random and suicidal. You won't be able to control them, and they'll run around as they please.

Are there any other changes to riding them?

You can now jump while riding a pig, using the same button as you normally use (for most people this will be spacebar). They also have a new sound for walking, and will consume the carrot on a stick as you ride it.