As above they can be great for semi-long distance sniping and are usually found on Ishtars or Domis. Drone Link Augmentors and a few levels in EW Drone Operation will help boost your range. They are also ideal for POS shooting compared to other drones as they don't get stuck on the POS sheild heh.

T2 Sentries have better DPS than the other drones, less likely to be lost to smart-bombers (as they are close to you, not running up close to the enemy ship) and of course you can give them a nice boost with the sentry drone damage rig. Also easy to recall unlike heavy drones who are slow as hell and easily taken out by enemy frigate hull pilots.

I usually carry a round of t2 bouncers or gards on all my pvp domis but it can be very circumstantial as to when they are brought out.

Also see:

http://skilltrainingcomplete.com/questions/786/sentry-snipah-maximizing-range

Sentry drones work very well with ishtars or gilas. You don't have to worry about having your ship within optimal range of your target, or how your transversal is affecting your own tracking.

Sentrys can also tank a decent amount of damage, take a long time to lock and can be scooped up and redeployed to break enemy locks. No one usually tries to kill drones, and even if they do, you probably have spares. They also provide you with a nice anchor to orbit around if needed, although this is more useful in pve.