Where can I find a quality comparison for Zwilling Knives?
There's a serious dearth of information about the differences between the Henckels product lines, but from what I can tell:


The biggest difference is the handle design, which impacts the balance.  You'll want to hold the knives to find which is right for you.
Stamped blades are used in the following lines: Twin Signature, Twin Gourmet, International Everedge, International Everedge Plus, International Fine Edge Pro, International Fine Edge Synergy, International Eversharp.  Source: Henckels Site.
Forged blades are used for the following lines: Twin Profection, Twin Four Star and Four Star II, Twin Cuisine, Twin Professional S, Twin Select, International Classic, International Forged Premio, and International Forged Synergy.  Sources: Henckels site, Squidoo, Amazon
Microserrated blades are used in the following product lines: Eversharp Pro, Everedge, Everedge Plus (Source: Henckels International website)
All but the Twin Cermax line are X50CrMoV15 steel, hardened to 54-56 HRC (some reports say up to 57 Rockwell C).  This means they're slightly softer than Wusthof (which now hardens many of their products to 58 HRC), but less brittle.
Twin Cermax uses special, superhard (66 HRC) MC66 steel, meaning they take a sharper edge and hold it MUCH longer than the other lines.  This is probably a clone of the ZDP-189 supersteel. (source)


The microserrated blades aid in slicing, but hurt the ability to chop and mince.  They WILL help retain a cutting edge longer, but also cannot be sharpened normally.  

Edit:
Personally, I've not been impressed by Henckels knives.  Wusthof knives strike me as better quality at a similar price, and Victorinox is almost as good but a lot cheaper.  Why do I say this?  Well, Wusthof blades aren't as thick and unwieldy as Henckels, and they harden the steel to 58 Rockwell, so it takes a better edge. Victorinox is comparable hardness to Henckels, but about half the price, and the shapes of the blades just seem so perfect, with a deeper belly to slice thick items, and awesome grippy Fibrox handles.

Edit 2: 5 March 2016  - Henckels appears to have moved to a 57-58 HRC steel standard now for many of their blades, and switched to a narrower 15 degree bevel.  This means they are capable of finer cuts, but the edges may be a tad more fragile. Twin Profection is now 60 HRC. This means that steel-on-steel, the knives should now be competitive with Wusthof. The Twin Signature and International Forged Classic still use the inferior 53-55 HRC steels. Victorinox has also raised the prices on their Fibrox-handled knives.  Personal opinion, they're still a much better value than Henckels, but Henckels may be more competitive with Wusthof now.