Qtractor is a professional-grade, meat-and-potatoes digital audio workstation: it features all the must-have features that you would expect from a pro DAW, excellent MIDI sequencing framework, and is actively developed in a sensible and stable fashion. It is written by the developer of Qsynth, Samplv1, Synthv1, and the JACK front-end that is so ubiquitous that it is JACK to many users: QJackCtl.
If you are a flexible and proficient DAW user, Qtractor will be familiar enough to you to pick up in a week or two. After you customise the keyboard shortcuts and make a few UI tweaks, you're a pro in under a month.
Heaps of features, with more appearing each version. Thanks to the communication that the developer has with his user base, the new features are useful and sensible, not just programmer-centric experiments.
Qtractor can handle both audio and MIDI, but its MIDI features are noticeably strong.
If you are new to professional DAWs, Qtractor may at first seem complex.
If you are an audio engineer and have no interest in MIDI sequencing, Qtractor may be too MIDI-centric for you.
Qtractor is a JACK-dependent application, meaning that it can render complex audio routing that could confuse new users.
The version of Qtractor made available by SlackBuild.org and other sites can lag behind the latest release. If you're just testing Qtractor, that's fine, but if you use Qtractor on a daily basis then it's worth getting the latest release for ony bug fixes or new features.
Qtractor can be installed from http://slackbuilds.org.
The user manual for Qtractor was written by the same author(s) who write Slackermedia. Instead of re-printing the manual here, grab the manual directly from http://sourceforge.net/projects/qtractor/files/qtractor/docs/.
Also available by the same author is a quick start guide written for Linux Identity Magazine (the article was written for Fedora Linux, but using Qtractor is the same).