Nuby sippy cups leak when you don't line up the notches with the rims and when you screw the lids on too tight. That actually makes them leak more. Also, they will leak if you depress the spout against something, ie. a diaper bag or if your child throws it on the floor and the spout valve opens. We always put ours in a zip lock plastic bag when putting it in a diaper bag just in case.
We have tried numerous brands of sippy cups and there are problems with each one. We still use Nuby the most. The Nuby cups are inexpensive and they now make the replacement valves and replacement spouts for the cups so you don't have to buy a whole new cup. They have also come out with a brand new sippy cup that doesn't leak. We have one. It is molded silicone, one piece, no notches to line up and it covers the entire lid underneath. It is BPA free, sells for about $4-5 in Walmart stores and online and eventually you will be able to buy replacement spouts if your little one decides to use it for a chew toy. To find these and the replacement spouts you can go to google and type in "nuby replacement spouts". Hope that helps.