I have a few thoughts for you:
First, according to what I read in various reviews, any quality rain suit will keep you dry. Hum... look for the word "quality" on the lable I guess.
I have three different types of rain suits. The first is the liner in my gear that is supposed to be waterproof. I find it leaks at the neck in heavy rains.
The second set is by a company called "RHINO." If I really tighten the neck, it is very waterproof. In the past I have put the Rhino coat over the "waterproof liner" gear and stayed dry even when riding for a few days in almost constant rain. This suit is made out of a thin vinyl like material, however and does not breath. After a day or two in it, I find myself chapping from being wet with sweat all the time.
Recently I bought a set of Frog Togs. The coat has an integral hood that goes under your helmet. The first time I used them, I didn't use the hood and water poured in around the neck. I got soaked. The next time, I used the hood and they worked well. The fabric does breath to a degree and I'm thinking they are going to be my favorite suit.
Nearly anything will keep you dry in light rain- heck the Tupperware does a reasonable job itself. In the heavy rain, (given that I realize its coming) I will continue to put the waterproof liner in my riding pants and coat and then put the Frog Togs over it. Probably overkill, but there you have it.