I actually got an after market gel case and happen to love it. Doesn't add too much but just enough to make it feel a little more solid. I like how the Xoom remains totally usable. I have the portfolio case as well and think it's great for protection but I don't like how it flops around when really using the Xoom and portrait mode is a bummer.
I like the gel case because I can easily use the dock, I can use ,a more sturdy than the portfolio case, tablet stand and I can easily lay it on it's back without a problem or without thinking about it. Also, I have the keyboard and the get case lets me prop it up anywhere because it's kind of sticky and that's been working out pretty good so far, especially at work.