While powdercoating does look great, a mediocre paint job goes a long way too.
When I bought Sandy back in 1999 she had a confer rack that had been sitting around in Monterey CA (coastal, foggy) for a few years. After light wire-brushing and two new coats of paint, it did great for us for a few years (including a year of travel), then it has been sitting by the garage in Northern CA for a few more years... and has some surface rust but nothing serious.
The rack I'm running now, we got it used (was at least 3 or 4 years old) and havehad down here for 3 or 4 years, sitting outside and has been repainted once with no prep work (just washed, left in the sun to dry, rattle canned black). It has some rust, but nothing that an hour with a wire brush wouldn't fix.
I guess that the moral of the story is that if you repaint every 4 years or so, yoru rack could well last longer than many other parts on your truck.
Of course, if you have more cash than time, getting a pro to sandblast/powdercoat is probably the *best* finish you can get for your roof rack.