And assisted GPS isn't necessary for a good fix. We use GPS devices on ships, in the backcountry, and on the track without it and get great results. That being said, one thing a camera needs is a quick fix; i.e. you pull it out, turn it on, and then snap, and turn it off. The trick for THAT application is to get your location quickly.
Frankly I too tried USB and BT GPS receivers and other solutions. And I have a GPS enabled Panasonic TS5, which is great. And I have a wifi enabled camera, an E-M10, which easily syncs with my iPhone and the Olympus app to get geolocation. But sometimes it's just as easy to use a decent tracking app on the phone (Gaia GPS or Motion X are my favs) and then either HoudahGeo or LR to sync up location info later. Or, if I've imported into the iPad, via an app there.
The nice thing about using a phone is that you don't have to continuously track (the phone may be doing it anyway, but you can just take waypoints every time you stop to take photos rather than making a track). This saves batteries. If you take a photo of coordinates and/or time on your phone, it's easy to sync up via software later. Or if in one spot for a while, just take a georeferenced iPhone photo and then copy/paste the coordinates in your favorite software later.