Since the very first radar gun and the very first radar detector have been invented, an arms race of sorts has began. Wielders of the radar gun, members of the police force, attempt to catch drivers who drive over the speed limit. On the other side, the average Joe, owner of a radar detector, does not wish to be caught speeding nor does he wish to be falsely blamed for speeding because of a faulty gun.
This arms race causes manufacturers of radar guns to find way of overcoming the ability of radar detectors' ability to detect them, and vice versa: manufacturers of radar detectors find ways of outsmarting radar guns - and even jamming their signal altogether.
Consequently, new models are occasionally released that are specifically aimed to deal with the new features of the 'nemesis' (for lack of a better word). Just like an old radar gun is almost certainly going to be useless against modern radar detectors, an old radar detector will not be effective against modern police guns.
The 'arms race' from a technical perspective
Radar detectors detect EM emissions from radar guns and trigger an alarm to the driver that a radar detector is used in the area. In order to outsmart radar detectors, radar guns change frequencies to those that radar guns are insensitive to and so, are effectively blind to them. However, once detector manufacturers figure this out, they enable their devices to be sensitive to these frequencies as well.
Furthermore, some devices passively absorb the radar signal, and consequently, render the car invisible to the radar gun. Note that this can look suspicious to a police officer with a radar gun.
New radar guns use laser light (called LIDAR) instead of EM emissions. Older radar detectors would be completely ineffective against this type of radar gun. However, radar detector manufacturers have a way to circumvent that as well.
In addition, technology has been improving in other ways. By incorporating GPS technology into radar detectors (as some model do, for example, the Escort Passport 9500ci) radar detectors do not always have to detect the presence of radar guns - all they need to do is connect to a database of people who have reported the presence of radar guns in certain locations, and simply drive slowly in these regions. This is an impressive feat, though personally I believe the other side is already working on a way of circumventing that as well - perhaps by creating numerous false alerts in order to make this database useless? We shall have to wait and see.
From an academic perspective, it is an interesting topic to follow. However, in practice, if you want to use a radar detector, you must be sure it can deal with modern radar guns, otherwise, you may be setting yourself up for a very unpleasant surprise as you might find yourself overconfident because of your radar detector device.
In conclusion: only by reading up-to-date Radar Detector Reviews you can be certain that you pick the best radar detector for your needs, and that it will be effective in this day and age. One last point: it goes without saying that before buying a radar detector, you need to make sure that using and owning a radar detector is legal in your state and country.