Well they still have a select number of stages that you play through, DeathSmiles being a little different and giving you a bit of flexibility when picking the order of the levels. Even though there are stages though there is tons of replay value to their games because you will constantly be trying to play the same stage over and over again to get a new high score or you might play from start to finish to get a new high score or to try without dying. Out of all the App Store games I purchased the Cave games probably get played most on my device.
To me, Cave games is either about survival (I like to play Bug Princess on hardest difficulty) and/or high scores. I like bullet hell games for survival. A lot of people like the high scoring aspect. If you like high scoring chasing, Cave games are for you. Dodonpachi, as I understand it, has a lot of unlockables/hidden stuff in it. Deathsmiles has a "iphone mode" which has an RPG lite element. You collect random item drops which has different status effects. You can replay a lot to collect all the items. As strivemind recommended, you should pick up the IAP item that increases item drops. If you like endless though and don't mind non-Cave, look up Phoenix HD..its free.
Yeah check out Phoenix HD, it's an endless randomly generated shump, and it's free. http://toucharcade.com/link/http://itunes.apple.com/app/phoenix-hd/id445791750?mt=8
