Of all the waterproof/breathable fabrics, eVent is the most breathable by far. Goretex Packlight and XCR are both very good with XCR being a bit tougher. Marmot's waterproof/breathable jackets also perform pretty well. There are a whole host of generic waterproof/breathable fabrics now in use and most of those perform about the same. Guess the right jacket depends upon your needs. If you are very active in highly humid conditions or a very heavy downpour, the advantages of waterproof/breathable may be lost.
Good waterproof/breathables vent away some of your perspiration when the humidity inside your jacket exceeds that outside. They won't vent all your persiration if you are very active. Pit zips, a front 2way zip, etc, can be the best way to vent excess perspiration.
Near and below freezing, they may not perform so well. Also, in a heavy downpour, they won't breath at all because the pours of the jacket are blocked by rain soaking the outside of the jacket. Here again, pit zips can come in handy.
If you expect waterproof/breathables to cut the wind, vent some perspiration under dry cool conditions, and keep water out when the downpour comes, then they are for you. If you want breathability in highly humid or downpour conditions, forget it. Buy a urethane coated jacket with lots of venting zippers and don't move around a whole lot.
Check these sources out for some other perspectives:
http://www.backpackinglight.com/cgi...tegory_display?cid=46&cat=Clothing - Raingear
and
http://www.verber.com/mark/outdoors/gear/clothing.html#rainshell
and
http://www.outdoors.org/publications/outdoors/2006/breatheasy.cfm
Find the shell that best suits your activity.