The latest versions of Cyberduck are quite nice, albeit slow at times. It's been very reliable in my experience.
http://www.cyberduck.ch
But, what you want is MacFusion:
http://www.sccs.swarthmore.edu/users.../MacFusionWeb/
The beta is SSSSSOOOOOOOOOO nice that I immediately ponied up $5 to see it come to completion. You must install Google's MacFuse first either via google
http://code.google.com/p/macfuse/
or via fink, if you already have fink running.
MacFusion mounts any FTP or SFTP account as a volume. And, it works as promised.
MacFusion IS beta! I had a kernel panic when I forgot to unmount an SFTP volume and changed networks. Otherwise, it has worked exactly as promised so far. This is the application that will make an (S)FTP client totally obsolete!
Payam