I love Spelling City and we use this site in our classrooms on a weekly basis. It is worth the subscription that we pay each year.
Recently, Spelling City developed an iPad app and so I asked my spelling groups to put it to the test. It was a huge hit and my students were impressed. The things they liked were that they could sign in and access their spelling list, the activities and games worked super fast, and they liked the sounds and animations with the games.
To explain more about what the app has to offer, I suggest reading Nic Peachey's blog post. He does a nice job explaining the app thoroughly and with screen captures.