I have a contrarian opinion about the impact of DARPA and related programs (ARPA-e especially). To some extent, there is a confusion of cause and effect. DARPA is good at identifying possible nascent technologies and funding them. They then have a flag planted, and can claim "we helped invent X", which is hard to disprove. But was it because of DARPA involvement, or was DARPA merely there at the right time? They have definitely funded plenty of projects that go nowhere.
ARPA-e makes this strategy explicit. They'll give money to an up-and-coming startup between rounds and then claim all later investment as "follow-on funding". They then report "follow-on funding" as a success metric to congress, which seems easy to game. ARPA-e has been around for over 10 years and I don't think they can point to anything coming out of the program as a legitimate game changer.
I do think the applied research programs do some good and has some impact, mostly as a way to convene the broader technology community. But for the most part I would rather have the government focus on funding basic research instead of applied research.