Pedals+Shoes are definitely high on the hierarchy of "stuff to get" after you get into cycling.
Efficiency is a big plus. Have you ever tried to punch someone who's wearing padding? It doesn't hurt them very much does it? It's the same way with pedals. Your regular shoes have a lot of padding so there's a lot of power lost when you're pedaling. Cycling shoes have a stiff sole with little to no padding so you get much more efficient power transfer.
Of course, from a regular fitness standpoint you can be just as fit without special cycling shoes...but the clipless system is definitely safer than straps. Especially if you're riding in slick conditions. When it's raining, your pedals can get very slippery and it's much safer to have the security of the clipless system. Also, when you're putting a lot of power on your pedals, such as in sprints, clipless pedals will be more secure.
Although Shoes are expensive, they will last you for a VERY long time. I bought a pair of SIDI Genius' for about $200 and they have lasted me for 5 years, and they can probably last me for more years easily. You can get cheaper shoes, but in my mind SIDI is a very excellent brand. Just make sure you read shoe reviews so you're aware of the design flaws/strengths of the models you want.
Pedals can be very cheap, but they can also be very expensive, $20-$500 depending on the model. Cheaper pedals will give you most of what you need. The more expensive pedals tend to be lighter and offer more tension. Durability varies a lot, though. I have LOOK Keo Sprints, but from what I hear they are not as durable as Shimano SPD's, even though they are more expensive. However, durable pedals will last you a very long time.
You will have to replace cleats. LOOK cleats ($16) don't really last very long (about 1000-2000 miles), but other brands have more durability.
Any cons other than price? You'll walk really funny when you're wearing cycling shoes. So if you have to be somewhere where you need your regular shoes you'll either have to bring them with you or ride your bike with them. Depending on the brand this may not be possible. I don't think it's possible with Speedplays, but you can use your regular shoes on pedals with a large platform, like LOOK. There are also models designed specifically for mutli-shoe use, so you can look into those if you need to.