I use a detector, and have done so for quite a few yrs now, it has definitely saved my butt a few times.
I dont know the specifics, but I do know Ive picked up a cop radaring from a good 4 miles out. and yes, that is the true distance because I watched my trip odometer for that very reason as I was wondering why I was getting a signal for no reason.
the rabbit theory doesnt always work, dependent on how many cars are on the road. if only 2 cars (you and the "rabbit") guess what, it isnt hard to figure out that your following at the same speed, however as has been mentioned, its hard to prove and most cops wont bother as it is a black mark on their report if you get a ticket voided.
around here you can usually get away with 9 mph over on highways, up to 15mph on the interstate, and 5mph on 4 lane suburban roads, but city driving you had better be doing the spd limit if you see a cop.
I also wish to address the comments about speeding and time saved. its all about the timing. where I live, if you do NOT do 5-7mph over the speed limit, you get nailed by the stop lights. I have it timed for the drive between the next town north of me (a larger town with several amenities this town doesnt have such as a home depot and taco bell
) the limit between the two towns in the rural area is 65mph, if I do 70mph I will make the lights on green, saving me a BUNCH of time as they are slow as molasses... if I have to do the spd limit then I will get stopped at the 1st of the lights in town, which will mean I'll also get stopped at the rest.... this is true for either direction Im traveling. if you calculate how long each light takes and avg it out to about 3 mins each stop, then factor in regaining spd as well as idiots who might not do the spd limit in the first place, or that accelerate at such a measly pace that by the time you actually hit the spd limit your 10 miles down the road.... then your looking at easily saving yourself 30+ minutes on the road. once your timing is ruined though and you get stopped at 1 light, your better off just doing the spd limit to save on the gas.
where I grew up, the timing of the stop lights all the way through the city was such that you had to do 47-48mph (posted 45mph) to make all the lights. Ive tested this time and time again, if I was forced to do the spd limit or a little under due to traffic, then I would usually make 1, maybe 2 lights (the 2nd one turning yellow as I got to it) but then I would get the light red and that would be all she wrote for saving time.