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I have long relied on Alt+Tab to cycle through applications.
Since upgrading to Unity in Ubuntu 11.04, I found that Alt+Tab has been very slow. Today, I found that Super+W, followed by arrow keys and Enter replaces this functionality and is more responsive.
Has Super+W made Alt+Tab obsolete?
Alt+Tab isn't slow, it's just that by default there's a 0.2 second delay, I imagine so that a quick Alt+Tab won't get slower by creating thumbnails of windows or altering window opacities or things like that.
Open CompizConfig Settings Manager (ccsm) (install if not already installed), open the Static Application Switcher plugin config, go to the Behaviour tab and fiddle with the value of Popup Window Delay.
It's 0.2 seconds by default which I found to be too slow. You can set it to 0 if you want it to be instant, or some slightly larger value as desired - after some experimentation, I settled on 0.05 seconds, which means that a quick Alt+Tab to switch to the next doesn't start hiding the other windows or opening the window switcher panel but just goes straight to the next application (this is particularly useful for comparing the contents of two windows without the flicker of the window switcher which I always used to get back when I worked on XP), but if I want to switch to any other window and want to see what I'm going to, there's no discernible delay.
Has
Super+WmadeAlt+Tabobsolete?
Alt+Tab is still the fastest way to switch to and fro quickly (between a small number of windows that is). Especially if you're working with only two. I do about five Alt+Tabs before I do one 
+W ←↑→↓ Enter or +W OmgWhereAmI Mouse Point Mouse Click. Not to mention that it's a quasi-cross-platform semi-standard adapted by many a distro, display manager and OS.
So short answer: No.
At least for me. They both have their uses and complement each other.
Chris Morgan pointed out in his answer here (please credit him for that, I just improved on the form) how to solve that problem. In shell or per link ccsm :
sudo apt-get install compizconfig-settings-manager
Then proceed as shown in screenshot:
Don't look at it like one is substituting the other. Think of it like Super+W is an alternative to Alt+Tab when handling multiple windows. Alt+Tab is still quicker for 2 to 3 windows.
A quick Alt+Tab for them and you are done. For more than 2 or 3 depending on the user, Super+W is faster since you can directly select the window you want and visualize it while doing it.
For the comments above Remember that:
Super+W - Select Opened Windows
Super+D - Go to Desktop (Minimize All)
Alt+Tab - Select from a group of opened windows
NOTE - They will sometimes not work (For example the Super combinations and their effects) but if anybody noticed, there has been more updates since Ubuntu 11.04 was released than the same amount of time for any other previous Ubuntu version. This somehow tells everyone the work that the Ubuntu Devs and community are putting into getting an excellent product. So be patient and most if not all of this tiny problems will be solved soon enough.
I had the same issue, not being able to switch tasks with x+Tab. Have Logitech USV keyboard, not shared with a KVM Switch. Curiously there is an 'F-Lock' button, (controlling standard F1, F2, etc or OEM pre-programmed stuff.
Once I turned off F Lock, Ctrl+Tab allowed me to tab between tabs in Firefox... but nothing else.
Alt+Tabis slow? Not just delayed in appearance? You can tweak those parameters as desired in CompizConfig Settings Manager (I reduced the delay time to about .1 seconds for myself).