I recently saw an ad on Craigslist from AOE 4x4 for Xterra skid plates. I used to have these exact skid plates - I purchased them from Hevak and ended up selling them to a guy up in Northern California because I wasn't ready for the added weight yet (didn't have my suspension mods in place at the time). I will say this - they are a quality product. My understanding is that they were built using a set of shrockworks skid plates as a template. They're pretty much an exact clone with a few extra holes poked in them for water to drain out of some places.
However, if I remember correctly, the price was almost the exact same that you'd pay for a set from hefty or shrockworks, including shipping, so why settle for a knock-off from a shady company?
My vote is for hefty, which is what I'm running now. The turnaround time is quick (like Jordan said, they usually have them in stock and they'll pump them out quickly if they don't). I'd avoid paying for the powdercoating, because it causes a delay in manufacturing (usually at least a month) and they'll just get all scratched up in shipping anyway (if not on your very first trail run after installing them). They really don't have to be pretty - just hit them with a few cans of rustoleum hammered paint and keep some on hand to touch them up once or twice a year.
If you aren't planning any suspension mods any time soon, you may prefer the aluminum ones to save weight. My understanding is that they're something like 30% as heavy but 70% as strong as the steel ones. Plus, aluminum won't rust and therefore doesn't NEED to be painted if you don't mind the bare metal look when you're looking at the bottom of your rig. A member on this site (fiercextreme) is selling a brand new, never installed set of aluminum ones for $500, and there would be no wait because he has them on hand now. I handled them about a week ago when I stopped by his house, and they're pretty impressive, and AMAZINGLY light weight. He's only asking $500 for them, and they sell for $700 new (plus shipping), so this is an extremely good deal. If I didn't already have a full set, I'd jump on it in a second.
Here's the for sale thread:
http://www.sccxterra.com/smf/index.php?topic=3623.0