First off, nothing ever lasts forever. The radioactive gas, called Tritium, has a "half life" of 14 years, meaning each 14 years half of it turns into Helium. This happens slowly, so the sights get dimmer each and every day after they were made, on a gun or not. After 14 years they will only put out half the light -- and you'll find them hard to use a lot sooner than that as they dim. Different makers give different warranted lives -- but they all use Tritium and it decays at the same rate. They guarantee they will be "usable", but that may be in the eye of the beholder. I doubt you'll be happy with any of them after 5-7 years.
Getting fresh sights is as important as getting fresh fish, and a freezer won't help. Avoid any sights that have been in inventory for years or on "closeout", and be wary of any used guns with them. If they are old they'll be a liability and not an asset.
They come in all colors, I've found only the green and yellow to be the bright enough, orange is only passable. (BTW, they really are the same brightness -- our eye just sees green better.) Some turn the little vials on their sides vs. the normal end-on view. I haven't tried any of these, so no opinion. My favorite -- a big green dot up front, two yellow dots in the rear.
You also need to consider how well they can be used as "regular" sights in the daytime. Some have a bright white ring around the tube to help this, but they are always a compromise -- always worse in daylight than plain sights.
An important caution! Most pistols don't have adjustable sights. Some gun makers get God to shine down on them, and the sights always line up with where the gun shoots when first assembled. Most have to adjust the sights. Horizontal adjustment is easy, just slide the sights in their dovetails. Vertical adjustment is harder -- the front site must be filed, or many use front sights of different heights. When you put Tritium sights on a gun, you get the front and rear sight heights they send, and only an expensive pro can adjust their height. You'll find many guns that don't shoot where the sights aim after replacing the sights with Tritium versions! The only sure bet -- send the whole slide and have Tritium capsules put into your old sights. (This also gets you fresh Tritium.) Some guns will allow this -- some are even designed for it, like H&K P7's. Many just don't have the "meat" to hold a capsule.
In the end, they really aren't all that bright, don't expect them to shine. They can just about be seen in the dark, and do dim with age as I have mentioned. My vision dims with age too, and I am farsighted, so they don't do a thing for me without my glasses on. But if it's pitch black and you don't have a light to shine on your target, it's the only way you can aim your gun. And now to debunk the promised myth.
A whole lot of experts -- magazine writers, cops, even cops who have shot people -- will tell you Tritium sights are worthless because cops don't use the sights on their guns. "Yes'ere, when you're in the heat of battle and the bullets are flying and your life is on the line, you just point and shoot instinctually." Bull hockey. Nope, I'm not a cop, only held a person at gunpoint once. But none of these guys have shot at people in the dark every night either. And one thing I have seen -- just about every dashboard-cam police video of a shooting, or someone being held at gunpoint by a cop. I've seen about 100 of these, and the officers are just about always professional -- they keep their finger off the trigger till they shoot, they watch their target background -- and they hold the gun up and look down the sights. All the "famous" tapes like the Ohio brothers shoot-out, the passenger who starts shooting, the tape of the three guys who try to take down an officer and he shoots at them as they run, and then into the field -- all these tapes show the officer holding the gun up high and aiming down the sights.
A quick summary: You won't aim in the dark without them, even can find the gun a bit more easily. But they are worse in the daylight, and -- like most of us -- get older and dimmer everyday.
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