MacHg is a fairly full OSX client:
It is a native OSX GUI client for Mercurial. It is modern and fully multi-threaded using Grand Central Dispatch and threading goodness. It has a clean interface and allows multiple repositories per document, using a standard mac sidebar interface. It incrementally loads data so its very fast. Ie it easily handles browsing the mozilla repository which is 3.35Gb. MacHg is fully featured handling all standard Mercurial commands, and additionally provides history editing features through the Mercurial rebase, strip, collapse, and histedit extensions.
I use Murky. There are two things to note:
When you enter the repository url use this pattern: https://username:password@domain.com. That's the only place where you can enter your username and password.
There are some things you can't do through the GUI (I think merge is one of them), in which case you can launch the terminal from the app and use the command line. You'll then see any changes update in the GUI.