On DSI synths I have found the headphone output to be lacking. The sound is somewhat thin and bright compared to using the same headphones through an external amplifier connected to the main outs. Switching outputs will probably not affect the decision of whether to purchase one of these units, but it certainly makes the sound more pleasing.
Also, these synths really come alive when tweaking the parameters. They respond in a smooth, dynamic way that I have yet to hear from any digital synth, which is more of a "sweet spot" hunt and preset machine. Coming from digital I was surprised how "alive" the basic sound could be, and how inspiring that was to sculpt and play. That is what convinced me.
If these units had sounded too similar to gear I already owned it would not have justified the expense. Then again, I could live happily on just the P6 and OB-6 (or either if need be) and use soft synths for everything else. I can't say the same for my other synths.