Of the four, Picasa, Flickr, and Locr read the geo-metadata of the images and displayed them without a hitch, whereas Panoramio failed to display my images at all. I liked the minimalist displays offered by Picasa and Flickr but, for me, Flickr had the aesthetic edge; the photo thumbnails on the Picasa map are a nice touch, but result in a cluttered view at standard zoom levels. Locr is capable of generating a slideshow display, complete with auto-generated location-specific facts at the top of the page. Very nice!
Each of these sites has an upload limit (monthly, total, or both), but each also offers additional hosting space and relaxed uploading limits if upgrading to a paid account. As far as I could tell, none of these services offered embedding of generated maps into a webpage, at least not directly. Also, each focuses solely on photos, compared with EveryTrail — admittedly, designed for a different audience — which is capable of displaying photos, video, and GPS track, waypoint, and altitude info in an embedded map.
Depending on your needs, and where you already host your photos, any of these sites are quite capable of displaying your geotagged shots (I'll revisit Panaramio — I suspect user error!).
For me, EveryTrail comes out on top. The site is well designed and intuitive, with an active user base, and the ability to host and map both photos and video is a treat. What really sets it apart from the other sites is its inclusion of GPS data on the map. Track and waypoint information provide much more context to geotagged images than simple location information as images can be associated with a particular trip and browsed in sequence. Move your cursor over an embedded map and you're presented with the option to display trip statistics: speed and altitude changes over time during the trip. Best of all, the site is free, has great support (that metadata problem I had yesterday? — fixed, thanks to the forums), and Chris McCarty wrote to say that they have no upload limits. Excellent! And if you own an iPhone, there's even more to love.