When you send the photos via e-mail, the recipient will receive them with a call to action to download the app. When you enter a phone number, the photos and download link will be sent to the recipient's SMS client. Once your friend downloads the free app, he or she will be able to browse and send photos just as easily.
Carousel also has an easy way to pick and choose several photos to send. While you browse through your library, you can drag pictures individually to a bar at the top. When you're finished selecting photos, it just takes a couple of taps to share them straight away.
Photos as part of the conversation
Carousel also acts like a chat client. Once you share a photo with another Carousel user (or group of users), you can chat using a familiar chat-client interface. But what's neat is how the app lays out multiple photos. When you select several photos to share over chat, it groups them together in thumbnails so your friend can quickly touch to get a zoomed-in view.
Also, when a friend shares a photo or group of photos with you, you get a button at the bottom of the thumbnails that lets you save the photos to your Carousel. Using these methods, you can share numerous photos incredibly quickly while talking about them, which is something regular chat clients don't do nearly as well.
Getting your friends on-board
The only problem with Carousel is that to get these easy sharing features you have to convince your friends to download the app and get a Dropbox account as well. It might be a tough sell for people when a regular SMS client will do mostly the same things (though not nearly as elegantly).
The other thing that will become an issue is when you run out of space in your Dropbox. Though 5GB is a lot of space, once you reach your limit, you'll have to either delete photos or sign up for one of Dropbox's larger plans with a yearly subscription.
Carousel offers great ways to share and view photos that are faster and more efficient than what you get on your smartphone now. The ability to select multiple photos and the way it lays them out in groups of thumbnails makes for easy viewing by you and your friends.
The only issues are getting your friends to download the app and the looming limit on your Dropbox storage capacity. Once you show how easy Carousel is to use with your friends, I think it might be easy to convince them. But once you cross that 5GB limit (admittedly a huge amount of photos), the tougher sell is going to be the need to sign up for a subscription.