Microsoft has posted a breakdown of Surface storage allocations between Windows RT system files and user-accessible storage, admitting that half of the available space in the 32GB model is used by Windows and built-in apps.
Outlined on the Surface support page, the FAQ document shows how 32GB in the decimal system equates to 29GB of total storage in the binary system. Once you remove 5GB for system-recovery tools and 8GB for Windows RT, you are left with 16GB in the binary system (or 17GB in the decimal system) to store music, videos and app data.
Those buying a 64GB Surface might not feel so hard done by, given that the same Windows install leaves 45GB (46GB decimal), according to Microsoft's maths.
This is, of course, what you would expect if you bought a new laptop, where Windows requires about 20GB of hard-drive space and this is deducted from the 500GB-plus sized hard drive that you have purchased. It's just harder to swallow when you start with so much less.
The Surface does support storage expansion via microSD cards. Just be sure you factor the cost of one into the final price before taking the plunge with a Surface.