23 is a lot like flickr. Almost a clone, even down to the UI and feature comparisons.
The service is free, for up to 15 MB of uploading a month. 29 EUR gets you an unlimited bandwidth account.
As to whether another photo sharing site can take reasonable market share now that Flickr is so entrenched – Sure. Maybe. This is still a massive growth market, and since 23 is located in Europe they will have an easier time getting users there, where Flickr does not have the market penetration.
Greg Yardley has more on 23.