Iris is probably both the most feature rich, and the most buggy of all the web clients.
Discussion
There is a high correlation between simplicity and stability.
Primal is the slickest/fastest of them all but that might be because 80% of the features aren't implemented yet. Feed loading is fast though and when you click the back button from a post back to your feed you always end up right where you left off. No jumping around like Iris does.