IE: My biggest complaint is that IE doesn't support tabbed browsing. Today, that's a deal breaker. I never managed to get used to the "Download Manager", either. There are other things I didn't like about it, but I don't remember what they are; I stopped using IE within a week.
Mozilla: This seems to be a Carbonized app. There are still some bugs to work out: my bookmarks got eaten a couple of times; it doesn't respect standard OS X idioms (Cmd-M to minimze the current window, bring back a window when unhidden). Most importantly, it's Carbonized, so I can't tell which LaunchCFMApp process it is.
Chimera: This is a Cocoa interface on top of Mozilla. Fonts look really nice when antialiased. I wish there were more control over some of the preferences, but that's a minor issue. The big problem is that it currently really spins the CPU, and it seems to be bounded by the number of open tabs in the relevant window. (The more tabs, the slower it gets and more CPU it uses to open/close tabs). Oh, and it crashes occasionally, too.
No browser is perfect; that's not a surprise. It's amazing that as complex as they are, any one of them works, let alone that so many work reasonably compatibly. But it sure is an interesting and unpredictable set of issues to be dealing with...