I want to use this primarily as a rain coat, is it light enough for that?

check out the Mountain Hardware Epic Jacket. just got it 3 days ago in time for the major Cali storm... lots and lots of rain. unlike goretex which requires a special "washing" (when you need to wash your funk out) with a special detergent and "follow up spray", the Epic is a WATERPROOF shell...no if's and's or but's. None of this "breathable" crap where you have to tend to the jacket to keep its waterproofness while maintaining a comfortable breathable nature. the Epic shell is extremely lightweight, due to it being a single layer of material *backed* by a waterproof plastic/rubber/nylon coating. It is able to compress into a small sandwich ziplock bag (maybe even smaller) to help with packing on trips. This coating on the inside ensures that there isn't a chance of water getting in... maybe if you ripped the jacket. The seams are taped and there are pit-zips to help with venting. The burnt orange colourway is awesome on the Epic... with a "ripstop" style crosshatches and a dark satin orange finish, the Epic was perfect for me. Check it out! In my opinion, there is no *waterproof&breathable* with the chance of water getting in. Pit zips do so much more in terms of vapor transport than a "breathable waterproof" material... unless you want to pay 3-6 times the price of a VERY good, simple shell.

Just got mine today, and yup, it's very lightweight, though there are other items out there where you could save 3-5 ounces. Pure shell this item.

Absolutely. I would highly recommend this jacket for any sort of weather protection.