This looks like a nice recorder, especially if it doesn't have any problems using a digital input (personally though, I never had any problems on my DR680 with a digi-in).
Hard for me to call it a giant killer though. Partially since there already are so many options out there for killing giants -- for example a littlebox preamp that really set the bar for price/performance ratio IME. Partially since the giant-killing option has already been there.
I've never run or even heard recordings from a USBPre, but assuming it is just like a 722/702 in sound: we've already had the option for USBPre> Sony D50 since the USBPre came out (as it provides an optical digital out). The D50 is only $130 more new than this new DR100mkII, so not a big change really. Plus I'd much rather have Lunatec V3>D50, which can be had (used) for just a couple hundred bucks than a USBPre>DR100mkII combo.
To me, the DR100mkII seems like it has a lot going for it though: low cost, small recorder; lockable XLR inputs, that take a hot line in signal (+24dbu); good internal battery management as discussed above; digital input (!!!!); can function as a good all-in-one option if the need arises or the user is happy enough with the sound; ability to be paired with high-end preamps and A/Ds.
The other good thing is that it really lowers the cost of a digital-input recorder, down from the 661 (about half the price?) -- particularly if the A/D doesn't have an optical out (like I said, the USBPre and opti-V3 users have already had the D50, but now there is a low cost recorder for those with stock V3's, AD2k+'s, Mytek, Minime's, etc). That is, meaning if it is reliable -- I know the Microtrack fits that bill, but I can't deal with its unreliability and general finicky-ness.