Slate is still very alive (and kicking), I just have been busy with my day to day job to make aggressive improvements lately. Plus, I did need to urgently catch up with Play 2.0 and Mongo DB so whatever free time I get these days, goes there. I intend to get fully active on Slate again by early June, though.
In the meanwhile, Eclipse based Scala IDE seems to have improved massively. It is much faster, smarter and does not hang up that often. I now prefer using text editors for writing Scala and Ruby though, so I am very excited about making Slate a light and usable development environment.
Early releases of Slate had a number of problems, especially in the UI. I believe most of them are now fixed, but there's still a lot I would like to do with it.