We have loved the 360 degree Munchkin cups for our littlest two. They are easy for them to use, easy to clean, and (mostly) spill proof! They also teach them how to drink from a regular cup. In a pinch, we can give our 18M-old a cup of water without a top and she knows how to drink out of the cup.
I just ordered 2 of the 360 cups for my daughter, but it seems she's a little too young to understand how to use them quite yet. They've come highly recommended to me though and I plan on trying them again in a couple of months!
I bought these cups because they were so highly recommended, too, and honestly could not figure out how the baby was supposed to drink from them...until he figured it out himself!!! It did take a little trial and error, though, before he just started sucking along the rim.