I bought a 16GB phone around 15 months ago. I thought 16GB would be plenty (it always was in the past, and I don't consider myself a "power user"), but here's where I am today:
Songs: 377
Videos: 17
Photos: 667
Applications: 51
Capacity: 12.6 GB
Available: 152 MB
i.e., I'm maxed out.
Going into the "Storage" menu: "Music" is taking up 2.1 GB, and "Photos & Camera" using 1.9 GB. At some point, I will offload all the pics/videos to the computer and start fresh; but they're almost all of our kid, and I like having them on the phone. (I upload the best ones to Flickr, but retain the originals.)
As for Music -- our iTunes library is so vast, I feel like I should have either no music on the phone at all, or like 10 GB worth. This "middle ground" -- picking/choosing just a few random artists to conserve space -- feels like it misses the point of synching with iTunes.
Bottom line: I would never go with 16 GB again. The iOS 8 update was a PITA. And as others have pointed out, software is only getting bigger...