I have used just about every name brand ballast out there. That being said, I prefer digitals, but there is a place for magnetics as well and I run both. Customer service is important to me and that is where allot of companies fail. For digi's Lumatek or Galaxy. I have never had an issue with a Lumatek and I have had 1 issue with Galaxy that was taken care of quickly. For mags, I like the Harvest Pro Elite's by SS. I used to like the CAP Xtreme's but a round of 2 dead ballasts out of 4 I just ordered, along with the fact that the order was placed on Jan 8th and I still don't have replacements in hand kind of put a salty taste in my mouth. I have had bad luck with the CAP digi's as well.
I wouldn't buy used but if you do, try to get a handle on the age of the unit and test, test, test before purchase. I tend to feel obligated to purchase when I show up to someones place off a Craigslist ad. I have also lost money on junk, including a supposed new digital ballast that was DOA when I got back to the lab. My bad for making the exchange in a parking lot at night with no way to test, but hey, the guy seemed honest, lol.