Greg Hauptmann wrote:
Just found and started to look at Streamlined (http://streamlined.relevancellc.com/). It looks very interesting however I couldn't find a mailing list or contact me link.
From their blog, it appears you can contact the developers via
streamlined AT relevancellc DOT com
 How mature is Streamlined? What sort of uptake does it have?
It is very new. DHH has just commented favourably on it, here:
Comments to his blog post were critical of the current look and feel, and the Streamlined team is addressing this by working with the developer of the Ajax scaffold.
If I remember correctly, Dave Thomas also mentioned Streamlined in his RailsConf keynote. He highlighted three directions in which he felt Rails needed to improve, one of which was high-quality scaffolding, and he mentioned Streamlined as a candidate for this.
The authors of Streamlined are well-respected people who have come to Ruby and Rails from the "Better, Faster Lighter Java" movement (Justin Gehtland co-wrote the book of that name with Bruce Tate).
So I would say: watch it carefully, try it out, if you like it and want it to move faster or in a particular direction, see if you can contribute.
 It it opensource?
Yes, MIT license:
(Note that the subversion repository is now accessible, despite what it says on the download page)
 Do people see it as a strategic add on to Rails at all (i.e. to complement rather than compete with rails)?
Yes, it adds to Rails rather than competing with it.