In lowbie, they're unstoppable killing machines. Absolute kings.
In level 50 PvP, they are very good. But nothing more than that. When it comes to killing the enemy's DPS, stealthers' number one priority should be hunting down the Gunslingers/Snipers. That's it, really. Sorcerers/Sages can wreck them pretty badly as well if there are options for LOS.
Even in sub 50; if you know what you are doing you can kill a sniper pretty easily with any class so long as you have an obstacle to work with. If you have that, you stay behind it when you see their big ol' batwings in front of them. Then when it is down you run up and knock them out of cover and pummel em. From range; you can use instant shots and pick them to death while LOS-humping.
Now in a crowded team fight a sniper can get you pretty good and you may not have the luxury of LOS