Don't think this was mentioned in this thread, but I have one of those squishy flexible rubber ones like this:



They're made by Adesso. They're not perfect, but here's my honest take.

Pros:
Super compact - rolls up to soda can size, or just folds over however you feel like.
Tough - not likely to break due to flexibility
Waterproof for the most part, resists spills.
Easy to clean.

Cons:
Have to be placed on a hard, stable surface to use, like a table, tray or large hardback book.
Feel of typing takes some getting used to (though not too bad, keys do move up and down)