I looked at some of them...a few seem good, a few are obviously amateur opertions. The thing with Sci Fi is that like romance, it is one of the few genres were the book cover is very important. They're collectible. I have an article abou that on my blog at
www.thoughtblog.net. Some of the companies have employed wonderful illustrators, which is pretty much the only good way to create a Sci Fi cover. Many use the Poser program, and while a few artists have mastered it, most haven't and the images look like computerized images (which might suit the book). Also, many of the images made with 3D software look super, super great online, but do not translate well to print, so you really need someone who understands colour and printing and presses. Nothing beats a good painting, though. Don't hand your baby over to just anyone...
Cathi