I've been messing around with the Cricut again this week, and let me tell you, it's a pain in the butt. Since I don't do many birthday cakes or cakes with writing on them, I'd be using it for patterns only. I think that I've been trying to talk myself into the idea of it being a useful tool, but the more I use it the less I can see having it be something that I'll use a lot.
In the meantime, I've used it on gumpaste, modeling chocolate, rice paper and icing sheets. Of those things, the icing sheets have been the clear winner in terms of ease of use.
Everyone is talking about gumpaste being the thing to use, but you have to roll it out perfectly thin, and have it perfectly dry (but not too dry), then you have to experiment with the speed/pressure/needle length so much that it takes too long to get everything set. There's a similar situation with the modeling chocolate in that it will rip if it's too soft.
The rice paper works well, but it has the disadvantage of being prone to curl when it's colored with food colorings. You can press it flat, though, and it does cut like regular paper, so it's good in that way.
The icing sheets have a plastic backing on them, so they can be stuck directly to the adhesive mat without worrying about food safety. The mat doesn't also get cut, so the shapes that you cut out remain safely on the mat until you pull them off. This prevents stretching and ripping, and you can store the shapes on the mat until right before you put them on the cake.
Overall, I'd recommend the icing sheets over any of the other media for the Cricut. I'm still not sure that it saves you any time at all, since you do have to sort of babysit the machine while it's cutting in case something gets jammed up inside it. It's not like you can walk away and assume that it will work perfectly while you're gone.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA, and cake supplies online at www.acaketoremember.biz and www.acaketoremember.etsy.com