We've used these cups for a long time now. They are very good at preventing spills, at least when the children leave the lids on. They also make an insulated version, which does a pretty good job of keeping the drinks cool.
Most of the problems we've had with them have come after our dishwasher broke. Since we've been handwashing them, the cups have had a tendency to grow mold in the lid and the plastic piece inside. Nothing seems to prevent it from growing - boiling or switching soaps. So we've ended up tossing a lot of them.
They do start leaking after a few weeks of use, most likely because we have to scrub the plastic piece so much to get it clean.
Overall, these are great if you have a dishwasher to clean them. We're still looking for a good handwashable leakproof cup.