Foam rolling is becoming more and more popular. They're very cheap and can be very effective when used properly and done at the right times.
There's nothing too magical about Foam Rollers. Essentially, they are very cheap massage therapists.
If you have access to it, you'll be much better off with a Physiotherapist or Massage Therapist to follow you round every day but Foam Rollers are a good, cheap alternative.
The Best Time To Foam Roll
The very beginning of your work out. Before you've stretched is ideal because it gets rid of any knots in your muscles.
If you have a recurring muscle injury, the chances are it's because of the scar tissue around your previous injury. When you reinjure the muscle, it's never in exactly the same place (although it may feel in the same place). It's always slightly above or slightly below where you injured it before. What happens is the scar tissue stops the muscle being able to stretch properly in that place. This means the sections of the muscle just above and below the injury have to compensate for that tightness and stretch further. Over time this leads to the second injury.
Foam rolling helps get rid of the knots and scar tissue and makes it easier to stretch the whole muscle and reduce your chances of injury.
If you're thinking of getting one for yourself, this one is almost identical to the one Cheryl is using in the video below.
This one is half the length (same width)
This one is for any particularly sadistic people (It hurts!)
Here's Cheryl showing you a simple routine you can do before every workout to decrease your chances of injury.