I just purchased Paws for Trello and am very impressed. Even though the basic Trello interface is the same (it's still the web interface), I like having a separate app dedicated to Trello rather than trying to find Trello in a sea of browser tabs.
Since it's a stand-alone app, the developer was able to add keyboard shortcuts…which I find very useful. For example, I can press ⌘⇧B to see all of my boards and ⌘+<number key> to quickly move between my favourite boards. Additionally, the app can be invoked through a global shortcut key and/or a menu bar item.
Native Mac Notifications are also supported and there are plugins for both Safari and Chrome that will automatically direct any Trello references (e.g. if you click on a link to a Trello board) to Paws for Trello.
Overall a great app that’s well worth the cost of entry.