Usability 
Pinboard is a good and mobile-friendly bookmarking service. The site is clean and simple and saving bookmarks is fast. The full Pinboard site at pinboard.in works great in the Android browser and with Opera Mini and Mobile and there's a lighter weight mobile version at m.pinboard.in.
Pinboard's features mirror those of the once popular Delicious.com bookmarking site before it was aquired by AVOS Systems and redesigned. Bookmarks include URL, title, tags and descriptions and can be public or private. Anyone, Pinboard member or not, can search public bookmarks or browse pages of recent and popular ones. You can share yours at m.pinboard.in/u:username. To share a single tag use m.pinboard.in/u:username/t:tag
Unlike Delicious, Pinboard is not free. There's a one time sign up fee, currently $10.09. The price is determined by the number of users times $.0001. That's an interesting pricing model. I can grok early adopters getting a better price. On the other hand the every increasing cost to join would seem to discourage growth - or maybe that's the idea.
I created a mobile-optimized Pinboard bookmarklet. You can find and instructions for using it here. It works with all browsers that support bookmarklets including Opera Mini, Opera Mobile, the Android 2.2 and latter browser, Safari, the WebOS browser, the Symbian Browser since S60 3rd ed. FP2, Firefox Mobile and Mobile Internet Explorer 9 and earlier.
Screenshots
