with a range of 50m, cant most players see and shoot much farther than that?

Is it only good for interiors, etc?

well it gives a bonus on perception checks, so a marine looking at an auspex is less likely to be ambushed by genestealers than one not looking at an auspex. Also it can detect colorless gasses, radiation leaks, possibly detect the weakest point in a fortification to place melta charges. its not really a radar, treat it more like a tricorder from star trek

In short I find them to be one of the most useful pieces of equipment

My players frequently use it to scan the inside of buildings or containers from the outside to check for nids. They've also gotten some mileage on the radiation and gas detection.

As has been said, the bonus to perception checks is very handy. It has saved the hide of some of my group on more than one occasion. And the ability to use it to look through walls has proven pretty handy as well, though so far in my group the detection of radiation/etc. has given hints and the like, but hasn't been as handy as the base abilities.

As Kshatriya pointed out. It can see through walls, behind bushes, buried in the ground, and around corners (within limits of course, your limits may vary, check with your local GM for variations and limits near you). So you can actually make perception and awareness tests on things outside your line of sight. Although you won't get heightened senses for these uses of an auspex. Makes sense though, you don't get sight/hearing bonus on things you can't see/hear, especially if you are staring at your auspex, but the auspex's in built bonus is good for this too.

You can also turn it on and leave it on. Unlike mind scan (which my group tends to rely too much upon, primarily because they don't use it correctly). Turn it on and set it to constant scan, any time anything enters into range of the auspex the user gets an opposed perception roll every time it moves (or 5-10s or meters if they don't stop, depending on your GM). If the auspex user succeeds in the test he knows they are out there. Meanwhile the mind scan is an instant thing, its not sustained, it only takes a snapshot. To get a good constant look your psyker either has to continuously use it every 10s or so. Necesitating constant focus power rolls, Either for perils if unfettered/pushing (with MANY opportunities to summon that daemon prince), or just to see if it goes off if fettered. Either way, the auspex is a much better option.