It really depends on the ISP. After spending some time trying to get phishing sources taken down and not getting anywhere, I wouldn't be hopeful about DDoS (reflection) sources being taken down either. When I was running servers that were getting DDoSed frequently (but thankfully for short intervals and not with tons of bandwidth), trying to get chargen servers or wordpress servers fixed didn't even seem like an option. Just make sure my servers wouldn't fall over, or at least would fall over gracefully.