The Next Gen still kicks ass. You can preview them on Netflix if you want. I'm running through the whole series right now. I understand not wanting to buy DVDs anymore, it feels like a waste of money if it's not primo quality. I was never into QL and never tried the latest Star Trek series because I heard so many complaints about it. When I'm done with Next Gen I'm going to watch a couple of episodes just to give it a chance but my hopes aren't high.
Other good sci-fi shows imo are The Dead Zone and Reaper.
Edit: Honestly though I'm not sure how much HD is going to help The Next Gen. It was developed for low res. Having the already iffy effects put into HD might just make their problems more pronounced. I would just watch them on Netflix and save your money. My 2 cents.