I had decided to spring for this unit as opposed the "660" because of the few [but amazing] reviews I had read (particularly the ability to leave it plugged in and it won't overcharge). The 660 comes up as #1 in regards to user reviews here on amazon, but that simply because of the NUMBER of reviews. This JNC950 unit has equally good reviews, but only 10% of the reviewERS. Well, I decided to spend the extra for this unit. It jump started my 91 camaro from a complete cold and dead battery - didn't have to hook directly to battery terminals either. It then started an old piece of crap camry several hours later. This thing was seam less - both cars cranked as if their batteries were completely charged.
if you're looking at the 660, but want something even more hardcore - buy this unit. It's worth it, it's not a toy (heavy as hell) and will jump start anything. no frills attached, no light, no nothing. just a jumpstarter.