I'm not crazy about the syntax and I avoid build.sbt files, but as a tool it's very pragmatic and takes care of my needs perfectly. It has incremental compilation and continous testing built in. For testing within SBT with something like say ScalaCheck simply works out of the box with no configuration necessary, by simply slapping a ScalaCheck test in the test directory. SBT has cross compilation support, something that is needed for Scala. Some of the best Maven plugins have been ported to it (I enjoy sbt-release for example). I tried my hand at writing a simple SBT plugin and it really isn't bad. Dependency management is based on Maven repositories, again something that I miss a lot on other platforms. Publishing stuff on Sonatype has been painless. SBT's support for sub-projects in the same repository worked flawlessly for me and it is easy to setup. The console support is great too, allowing one to easily open a Scala console with the right classpath loaded (compile/runtime versus test). And in general I like how it just works out of the box.
For Scala, the only fair competition that SBT has is Maven. And the only (other) pragmatic build/packaging/dependency management tool that I used has been Clojure's Lein, but lein lacks for example the capability of keeping a JVM process loaded, so you suffer the startup time every time you invoke a command, plus it seemed weird that lein doesn't offer things like continous testing out of the box. And speaking of configuration formats, lein's profiles are simply confusing.
You don't have to use SBT if you don't like it. Maven works fine and SBT's secret sauce, which was the incremental compilation, has been extracted in project Zinc and so it now works with Maven. But I still prefer SBT over Maven and when switching to other platforms it now feels like going back to 1999.
And for those working in Scala and building with Maven still, I second the recommendation to download & run Ivy, turn on the useZincCompiler option in maven-scala-plugin's configuration, and enjoy substantially faster build times (and automatically falling back to a normal build if Zinc isn't running). You don't even need to specify a scala home directory for Zinc like it claims; scala-maven-plugin will tell Zinc to use the Scala libs from your Maven repo.
I like this a lot. Setting up your own Maven repository on your own server, that can be internal or whatever, can be as simple as setting up a server that serves static files and the publishing itself is just copying files by means of SFTP or whatever. The protocol for finding all the listed dependencies on that server is also simple, so in case of problems you can usually understand what's going on.
This is much saner than the alternatives I've encountered on other platforms.