You'll know you're on an instagib server, if you're confused (the first time, almost everyone is) because you start with the
ASMD, it has no alternate fire, and it's normal shots are
orange. And, of course, they will
explode people on contact. :)
Advantages:
Weapons, and the stupid cheap tricks that ensue in the places where they appear, are no longer an issue. You have one gun, and it's just fine. No more running around dying until you find claw your way to the place where the decent gun spawns. The game becomes more focused on aim, reflex, and other "pure" combat skills than on item tactics.
Disadvantages:
As with all one-shot-kill weapons, ping becomes vital. People with low latency connections to the server will be able to aim and hit other players significantly more easily, since the other players will appear on the screen much closer to their actual position. All but fearfully skilled modem players will therefore tend to eschew this style of play, since it exacerbates their natural disadvantage (people are never quite where they appear to be), in favor of normal games and splash weapons.