I have a Radial Workhorse Sixpack with the following:
-2x CAPI VP26's with Red dots.
-2x DBX 560a Compressors.
What did you want to know? Having the ability to share a power supply saves space and more importantly, $$. Could not be happier with my CAPI's, will be building a pair of VP28s in the future. Lot's of great modules out there.
Just as an FYI I run it into an RME FF800. Good conversion and i/o is key to making good preamps shine.
I will go 500 some day, but for now my SSL and Neve channel strips give me everything I need for single-instrument mic'ing, and I use an OctoPre for multi-mic scenarios. For compression I go cheap n cheerful, though the Lindell stuff looks very intriguing.
what's it for, mainly recording stuff? outboard mixing? guitar use? I like the idea of having a bunch of nice compressors and such in one box (not that I need it). could you build a bass amp or preamp of sorts with compressors, effects, and a sick preamp module?
500 series is for modular studio effects.
You can mix and match to get your ideal channel strip.
It is used less for outboard during mixing unless it has a built-in summing amp like the Radial or Heritage units; unless you are using outboard for parallel compression. As far as I know there is not a 500 series module with reverb.
Lots of options available module-wise, from mic preamps, compressors, EQ, to I/O interfaces, phase alignment, etc. etc. I use mine mainly for tracking but will throw certain sources through the 560a's when needed.