I've been using ab (apache benchmark) for a few years, but haven't ever really known what I should consider a good response time.
I just ran it against my latest project, and did:
ab -kc 50 -t 60 http://thingist.com/
and ended up seeing an average response time of 2000ms
Which is certainly much higher than I would ever want under normal conditions (although the site stayed up, and I was able to post while the test was running).
Things like:
ab -n 100 -c 10 http://thingist.com/
usually clocks in at around 300ms
What do you use? How do you know if you're doing well?