Yahoo Answers internauts debate the origins, the appropriateness and the coolness of the term, using such language as "[People] say sammich rather than sandwich because they have never been taught to speak or enunciate correctly," or "Just try saying it, it makes me laugh". Some people love the word, others hate it.
On the "authoritative" website Urban Dictionary, you'll find a number of definitions for sammich. All of them, of course, note that sammich means sandwich, but most of the definitions also restrict the use of the word sammich to particularly elaborate, large or extra-delicious sandwiches. For example, "A sammich is not just a sandwich, it is not just a meal. Sammich is a
term reserved for only the holiest and mightiest of all sandwiches. A
sammich is a true work of culinary art; a feast on a bun, if you will," or "Sex in sandwich form. An orgy of flavor. A god among sandwiches."
We here at Flavors of Brazil deny ever having used the term sammich, and don't find it particularly cute. But the word comes in very useful when trying to translate the Brazilian Portuguese word sanduba into English. Sanduba is a Brazilian slang term for sandwich, but like sammich it carries connotations of being an extra-special sandwich, something above and beyond the norm. It's likely to be big, have a number of ingredients and is likely to have engendered enthusiastic reactions among those who've eaten it.
As is often the case with trying to use slang in a second or third language, if you're not Brazilian it's probably best to leave the word sanduba to the locals. Slang often carries connotations that unknown to foreign speakers and it's very possible that you might use the word in the wrong context, or with the wrong people, or at the wrong time. So, just to be on the safe side, if you're a tourist in Brazil, stick to the word sanduíche (pronounced san-du-EESH-ee). You'll be understood and you won't make Brazilians laugh at your or think you're rude.