I've tried several times in the past, putting serious time into getting a working MIDI based DAW setup going with just opensource stuff. I tried Rosegarden, LMMS, etc. My feeling is that the foundation, on the OS side is pretty solid. JACK works great, OS itself isn't full of crap.. But the DAW side was always a problem. Rosegarden was kinda clunky, LMMS is more of a fruity loops clone and I can't work in the fruity loops paradigm. VST support is also an issue since most opensource DAW don't have it on by default, although you can get the SDK from steinberg and compile the DAW yourself with VST capability turned on. Bit of a drag but not impossible if you're familiar with linux. (where compiling on your own is generally more common compared to other OS)
A few nights ago, I came across a sequencer/DAW called Qtractor. The interface seemed familiar so I must have checked it out a while back, but I thought why not give it a go. I got it set up with all the LADSPA/LV2/DSSI plugins I could find from package manager, and started messing around. There was some trouble in telling Qtractor where the plugins were, but once that was done, everything worked. (I didn't mess with VST integration)
Ironically, this was the first time I've ever seen a DSSI synth in action. I've never gotten them to work in a DAW before. The interface is all ugly and slightly jarring to use, but hey, they're still knobs and faders anyway so they're "workable". I tried the handful that I had access to: Whysynth, Xsynth, DX7 clone, and a couple more. They all sounded pretty good to me.
My first reaction was to start making some drums sounds to lay down drum tracks, but unfortunately these general purpose synths didn't have the right EG (decay times are very slow) and also mod routing capabilities (add 2nd EG for "snap" on kick) to do good drum sounds so I had to give up. They're good general VA synths tho, structurally very versatile and capable. Anyone recommend a good drum synth that's ladspa/lv2/dssi? Maybe I should just build me one in Alsa modular or Om/Ingen?

Just looking at this screenshot of Om/Inget turns me on.
Qtractor itself was nice to work in. Since I couldn't get drums, I didn't get into heavy sequencing, but just tried recording a few tracks, move clips around, basic editing stuff like that. For me, Qtractor reminds me of Reaper. It's sort of bare bones and overall feels feature light (might not actually be, but "feels" light) and a very no-nonesense interface that's all business. I still need to give it a serious test drive, but at this point, I feel I can actually get some work done in Qtractor. Big improvement from all the other trial runs. Of course, the problem of many of the ladspa/lv2/dssi plugins (effects) not sounding wonderful still remains, but I can just use VST. Although I'd like to see how far I can push the opensource ones. They're not horrible, they're just not impressive.

Qtractor looks like the common denominator of all non Live inspired DAWs.
So, things are starting to look up for my linux DAW dream. Definitely won't compare just yet to my Live/VST setup, but seems like a worthwhile tinkering project.