We started with Onyx (back on a HP DesignJet 8000), and absolutely hated it. The software was problematic, support was awful, and it was buggy. Colorburst was actually a huge improvement over Oynx from reliability, configuration, and since they OEM'd it for Epson, the profiles were great! However my one big BUT, was both use the JAWS rip engine, which cannot handle transparency properly -- ie InDesign type with drop shadows. They won't do it right, and you would have to drop those files on Photoshop and JPG-it to print it right. Massive time suck. ...and now-a-days, everyone seems to be using those little handy-dandy tools that are in CS3, CS4 and CS5+.
SO - we switched to Caldera. Their old version with the Ghostscript rip worked great, and now their version 8 with the APPE (direct PDF rip - bypasses Postscript) it not only screaming fast, but has been literally 100% reliable. We are yet to have a problem with the actual rip. Albeit, their interface takes some getting used to - the French just do things differently
-- but there is really no comparison between Onyx or Colorburst and Caldera. I would highly recommend Caldera to anyone in the market for a rip. Its pricey, but well worth the extra money. If you do any volume, you will get a ROI quickly (in our case, it was about 2 months, and it paid for itself).