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I am planning a reasonably large web application with CMS functionality and see Umbraco been the perfect platform. I have used it to build quite a few websites in the past and from this have also experienced some negative aspects when moving from v3 - v4.
Will moving from v4 - v5 be simple?
The application will include custom sections (main concern) and probably won't be packaged as this seems like a complex process even from v4, correct me if I am wrong.
When version 5 was mentioned at CodeGarden this year it was stated that version 5 will NOT be backward compatible with version 4 and there will not be an automatic upgrade. Version 5 is the MVC version of Umbraco where version 4.x is the web forms version. But....they are planning to continue to support Umbraco version 4.x, even after v5 :)
For some recap from a code garden presentation about umbraco 5 vs 4.x visit:
There are a few powerpoint presentations. Hope that helps in the decision.
(There's also a really awesome screen cast! :P)
To keep up to date with the progress of Umbraco and the timelines make sure you keep an eye on the Roadmap - http://our.umbraco.org/wiki/about/roadmap
Thank, I now work freelance and will be dedicating a lot of time and resources to building sites, apps and packages around umbraco, hopefully I will make the core team one day :)