I find Garmin's data and charts very useful. I've never been an "instinctive" runner: I don't know what I'm capable of, and I don't exactly know how I'm feeling past the basic post-running feeling... strange, I know. So I need to see the data to get a clear picture. Using the device's data has taught me a lot, and has made me connect the dots between my training and my performance. I fully credit my improvement in running (pace, economy, endurance, etc) for the past few years to Garmin's metrics help.

That said, it takes a while getting used to them. Personally, I use the data as a guide, but I never take predictions and such at face value. They can be a bit off, either positive or negative; I've seen both. For example, I think it tends to overvalue long distance training runs for performance prediction.

But I think I'm at a point that I can understand what it's trying to tell me, and find some correlation with how I actually am. I think that's the way to go with Garmin.