Plenty of times, they're a handy tool for recuding the amount of paddling you have to do while on a surfboard.
They're a pain in the ass when you're swimming, though.
People seem to think that a rip current will take you miles out so sea, but they wont. Here's how they work.
Sand bars build up along the beach. There start near shore and go no more than a few hundred feet into the water. Now, the water is shallower over the sandbar, and the when waves get to this shallow water, they break and and the water from the wave washes over the sandbar to the beach. There is now a bunch of extra water near the beach, and since water will find it's own level, this extra water needs to flow back out to the ocean, outside the sandbar where it came from. Since waves keep pushing new water over the sandbar, t his extra water will flow parallel to the beach until it gets to the channel between sandbars. this channel is deeper than the area above the sandbars, and so waves dont break here until they get right next to the beach. When the water reaches this area, it can flow back out to sea with out waves pushing it back up towards the beach.
This is how a rip current is created - all the water pushed over the sandbars by waves flows back out in a narrow channel between sandbars. However, once the water gets past the sandbars, which usuall doesn't take more than 100 yards, it flows to the rightor left and dissipates.
If you were to get caught in a rip current and jsut tread water, you'd get pulled out to see about 50-100 yards, and then you'd move parrallel to the beach for a little while until you were in front of a sandbar. At that point you can jsut swim straight in with the waves. Theother option is to swim sideways whe nthe rip current first starts pulling you, until you get out of the channel between sandbars.
The big mistake people make is trying tho swim against the rip current. They end up swimming as hard as they can, but not really moving, and eventually they tire out and can drown.
Rip currents are very manageable if you understand them and can swim 100 yards and tread water.