I own 2 nextgen 400/600w switchable ballasts, and have used them for a complete grow. They are very convenient and save a lot of money not having to switch specific ballasts for any reason. It's awesome. As far as performance, they are slower to warm up than other ballasts like a Phantom, which I would suggest over NextGen, because they are dimmable, which is as good as a switch, if not better, because you can dim down to a 250 if you want. My parents use Phantom and I'm jealous as hell.
MY MAIN PROBLEM WITH NEXTGEN
I purchased 1 open box from IE HydroFarm, and no problems at all. I buy another one brand new from IE HydroFarm a few months later, and it breaks within a month. I have to bring it back to Discount Hydro myself unless I wanted to wait 6 weeks, and I got a refurbished unit, which had the same random turning off problem as the first one. So I had to go back to Riverside to switch it out again, because I have a garden in the dark at home. They switch it out with a new one for fear of losing my business. Since then not a single problem and they work great, even if you have to turn them on and off through out the day at multiple intervals, if you get a good one they are a real work horse. But they use fans to air cool, and moving parts and flowing air means dust and motor problems that can occur, probably because of the fan or some foliar spray it sucked up, so beware of where you place them and that fact when you purchase.
If I had advice it would be to get a Phantom ballast because they are reliable and affordable, and if you cant get one, get the next gen 400/600 switch, the extra 30 bucks is worth it over the 600w ballast. But do NOT get a Lumatek, most returned ballast in the industry.