Big thumbs up for Mendeley. The most important features of Skim (high-lighting, notes), plus bibliography, intensely cross-platform, and in the cloud. I'm using it on an iphone, ipad, mac, and an NMCI Windows XP box (supports proxies). Oh, yeah, it integrates with Zotero.
Is it better now? I have tested it before but it was too buggy for serious loading (PhD thesis), although I liked very much the interface and features.
The in-application browsing experience is pretty lame on OSX and Windows, but 'open externally' works great.
The iOS app is almost there. Once it has sync like the desktop (right now you have to individually download each!) and supports opening in GoodReader so there's good display and editing, it will be useful. But for now, if you want to do any practical device reading, you'll probably want to pull it down on the desktop and transfer into GoodReader via iTunes.