I offer up the tank above as a simple example of what I've learned over the last 5 months. I started weathering at the beginning of this year and they really are super easy to use
In general, I really like weathering powders, whether you get them from Forgeworld, MIG, or someone else. I think I found some from AIM products at my local train store. A note of caution though, I would experiment a little bit to understand:
1. What brushes are best (I stole make up brushes from my wife)
2. How sealer will/can change the color of powders (my figures tend to get darker, so I compensate accordingly)
3. How you can combine powders with other materials (e.g., my mud recipe) to get nice looking, but very easy to execute results.
My advice, invest in a couple of powders (2 tones of brown, a rust, and black) and then invest the time to play around with them a bit. I think you'll be happy with the results. I know I have.
Here's my basic process for mud:
http://www.kan.org/michael/mkp/weathering_mud.php