I got one for a present. Men's size large weighed just 4.5 oz!!! However, it failed a quick, 5-min shower test miserably. My hair was completely wet (worthless hood) and my tee shirt was completely soaked through on the shoulders and the front. The back side fared better -- it was "only damp". If I were out in the cold, the soaking would be more than enough to cause hypothermia. I purposely kept the test short to make sure that perspiration wouldn't be a factor.
I complained to SD, mailed the loser back to them, and got a brand new replacement. I could feel the thicker PU coating immediately. This jacket weighed 5.5 oz or a whole ounce more. But hey, it would still be worth it if it's truly waterproof, right?
The replacement jacket still failed -- although it performed better than the first one. This time, the hood kept out the water, and my tee shirt was only damp rather than soaking wet. However, the two pockets (zipped up tightly) still let in enough water to form two water balloons!
Verdict: Ignore the seductive siren calls. Save your money or just buy any one of those UL water resistant wind jackets. They are cheaper, they are much more breathable than the Isotope, and they provide just as much rain resistance as the Isotope. Alternatively, take a look at wp/b jackets in the 7-8 oz range: GoLite Virga or TNF DIAD. No direct experience with either, but surely they can't be as bad as the SD Isotope! Shame on SD for both marketing hype and shoddy product in this instance!