No Scala functionality
If the Scala functionality appears to be non-functional after installing the Scala IDE and restarting Eclipse, in particular if Scala source files appear to be being interpreted as Java (with a consequently large number of "Java problem"s being reported against them, then please ensure that JDT Weaving is enabled by navigating to
"Preferences → JDT Weaving" then, if disabled, click the button to enable and follow the instructions to restart Eclipse. If this doesn't resolve your problem, check that JDT Weaving is still enabled after the restart.
Eclipse issues on Ubuntu (version?) with the default JDK (version?)
As of (date?) on Ubuntu, Eclipse has issues running with the default JDK. You should make sure it runs with Sun's JDK, see instructions here and/or here. Eclipse users on Ubuntu report that this significantly improves performance and stability.