Having done several weeks of pool-training behind me, I can say with certainty that you don't want to be pool running for long periods of time, as it can be about as tedious as running on a treadmill.

That said, there are supposedly numerous benefits to pool running. If done properly, and that's a big if, it can help improve your running form and strengthen your core and hip muscles in ways that running cannot. Additionally, it is reasonably running-specific since you are simply making running movements in a pool.

There are, of course, obvious differences. There is no impact in pool running (good if you're injured), and the muscle activation is pretty different than running. You'll find that your quadriceps and calves won't be getting the same use as in running (which again, can be a good thing if you're injured).

To pool run, I would suggest using the aqua-jogging floatation devices. Buoyancy varies from person to person, so while some people are capable of running without the belt (they float just fine), most runners will probably benefit from using an aqua jogger. I've heard quite a few people claim that you get a better workout without the aqua-jogger, but the truth is that while you might be expending more energy and working harder without it, your form is probably completely horrible and you probably look more like you're doggy-paddling than pool running. Form is the most important thing to focus on when pool running, so don't sacrifice form just to be getting a better workout (after all, you could just be swimming or doing some other activity. If you're pool running, it should be that you're simulating running to a certain degree.)

Anyway, here's a good resource for pool running:

http://www.pfitzinger.com/labreports/water.shtml