Photoshop Express requires a broadband connection such as DSL or cable. I worked with it under a 3 Mbps DSL connection and most operations were fluid and quick. Some operations, such as saving edits or building thumbnails from Picasa, did require a bit of a wait.
Photoshop Express works within any browser or platform as long as Flash Player 9 is installed. I worked with it in Firefox 2 and Internet Explorer 7, and the experience was identical. At launch, the service is only available in US English, but will be available to a global audience later.
Currently there is only one service level, which is the free account with 2 GB of storage. In the future, additional subscription levels with added features and storage levels will be offered. Photoshop Express does not display advertisements at this time, though it may in the future.
Photo printing services are not yet offered, but Adobe has stated it is on the roadmap for this year. Integration with other Adobe desktop applications such as Photoshop Elements, Lightroom, and Bridge is also in the works but most likely would not be seen until the next product cycle of the desktop applications.
Adobe is actively seeking feedback on Photoshop Express and plans to evolve the product based on user feedback. So go ahead and sign up for your free account, give it a whirl, and click the Feedback link at the top of the screen to tell Adobe what you think and what you'd like to see next.