I've used IceClean since early on when it was first released, and generally speaking I think it's a very good tool. Much like Onyx, it has most of the typical maintenance tools most users may ever need, along with a few extras that Onyx is lacking, though IceClean too is lacking a few extras that Onyx offers. As a system maintenance tool, some of the extras that IceClean offers really are out of place. It's not the type of app that I would expect to do a traceroute, whois, and other internet lookup tasks that have zip to do with my computer.
One big problem I have with IceClean is the interface. It's just really bad. For starters, all of the system maintenance tasks are run from the menubar, and is lacking in any explanations of what the tasks do before run. This is a fine option for automation because, unlike Onyx, all of those tasks can be run with keyboard commands. If one desired, you could very easily use Apple's Automator app to launch IceClean and run some tools at specified times/days without being present. For the average user though, Onyx's interface is friendlier with the tools right there in the main window with explanations of what many of them do. And speaking of the main window, the tools that are there are mostly tools like Lookup, Netstat, Whois… tools that, as before, aren't the central function of this app. It's just very out-of-place.
IceClean also doesn't do well with providing a clear indicator that a task is underway, or how far it has to go. You get a subtle spinning wheel in the bottom corner, and sometimes a sound when the task is done. That's typically it.
So when it comes right down to it, I see IceClean as a tool better suited to more experienced users who are comfortable with what the tool does and want to take advantage of the greater ease IceClean offers in the sense of automation and keyboard commands to run the tools it has. With some more work and extensive refining of the user interface, this could in time become the must-have tool for novices and experienced users alike.