Hi There,
I hope I can offer a bit of insight. (I am in no way affiliated with Roland or Yamaha)
Roland products, in general, are a bit $ inflated, you do pay for the name, and you get a very high quality product(usually). In the case of the HD-1, in addition to Roland's normal inflation you are also getting a "mesh" snare. That alone doesn't make it worth the price! When A/B comparing e-kits especially Roland/Yamaha I feel Yamaha gives you a lot more for your money. Yamaha also offers quality that meets or exceeds Roland's. (yes I am a dealer, so I have seen what goes out and what comes back) I suspect Roland knows this too, as they recently lowered the prices on the TD-4&TD-9 lines. For every 10 DTXplorer's I have sold, I have sold maybe 1 HD-1? And which come back time and time again? Not the DTXplorer or any in the Yamaha lineup.
That said, the DTXplorer has a lot more to offer than the HD-1. More triggers(yes there is an additional in on the kick tower), more kits, more sounds, more flexibility with placement of pads and much more. It also comes with everything you need to get up and running, including a real kick pedal and tower vs the HD-1's fixed foot switches. For an inexpensive entry level e-kit the DTXplorer has a lot to offer. Don't take my word for it A/B the two and you will see. If the "mesh" style head that Roland offers is the reason for staying away from Yamaha, know that you can add any of Roland's Mesh pads to the DTXplorer.
As far as people trying to sell the DTXplorer at 600.00 US it had better be brand new that is the MAP price on that kit. I wouldn't pay anywhere near that if used. Used in good shape, $300-350 US ballpark. I have seen them as low as 200 US in great shape.
Hope this helped....
Happy Drummin'!