I have both the 8GB and the 16GB and once fully set up with basic apps the 8GB has about 4gb free space. If I reserve 1.5gb for future apps, updates, downloads, texts, pics, video, photo etc. That leaves a working budget of around 2.5gb. Which is enough for the contents of my ipod nano and a couple of TV shows in h264 and podcasts etc. All docs and stuff go on google drive anyway ..
In short yeah its plenty if you're not going to be installing games or larger apps.
The 16GB has 12.5gb free after setup and half a dozen commonly used apps off the appstore. Which basically allows me to load it up with a few hrs of TV shows and maybe a HD movie or two for when I am on the road. I wouldn't say its really neccessary to get the 16GB model especially if you plan to use a USB thumb drive for media. Its just a little extra future proofing if you're going to have the G for a few years and if you plan to play any games you will want the extra space for sure. The price of the 16gb upgrade is some ?40 in the UK which is a bit of a rip off considering thats almost 1/3 the cost of the device again! Definitely a gouge!
I also often record my mumble conversations and group meetings for work etc. Which eats up a GB or so ..