I've finally put my money where my mouth is and have been using guar gum instead of xanthan gum in my baked goods for the past few weeks. Xanthan gum is corn-derived where guar gum is not. I can see no difference between the two in my baked goods. I've been using 50% more guar gum as xanthan gum, so if a recipe calls for 1 tsp. xanthan gum, I use 1 1/2 tsp. guar gum and everything has been turning out great. The one exception is yeast bread - for some reason guar gum is not working in my sandwich bread - read here and here.
Guar gum is much less expensive than xanthan gum, so even if I was not concerned about xanthan gum being corn-derived, I'd use it anyway because it's half as expensive, maybe less. Which really counts for something in my household these days! I am still using xanthan gum in sandwich bread but in everything else I'm using guar gum, and over time this will help keep the cost of gluten free baking a little more manageable - every little bit of savings counts!
Check out the comment from Niksmom (thanks for the heads-up, Niksmom). I have stated elsewhere that some people may have problems with guar gum because of the high fiber, but I had forgotten that legume sensitive individuals may need to avoid guar gum as it is bean-derived. For people who need to avoid legumes, see this helpful list of legumes here.
Julie - girl I am so sorry. Samuel cannot have gluten, corn, legumes...your gluten-free baking just got a lot harder. I will look up SCD for you. In the meantime, see this article about cross-reactivity, it might be helpful for you.