+1 to what Quinma said, but I'd like to add:
Sentries are actually not terribly effective against skulks. The reason is that sentries only attack in front of them, so if you can just jump over them, they can't hit you. Additionally, sentries require a nearby sentry-battery to operate. If no sentries are facing the battery (which is often the case, since they can't shoot through it), then you can take them all out at once by simply jumping over or around them and attacking the battery.
I find that sentries work better at gaining time: if a sentry is aimed at the power node, then the melee-enemies (which most of the aliens are) will need to take the sentry out before they can attack the power-node. This gives the marines time to make it to the room to protect it.
Also, if you're a marine and you're being attacked, running in front of a sentry could save your life.
Whips also are usually pretty easy to take out, due to their limited range. The only time they really cause problems is when they're placed around a corner - in that case, either try to run past or take a different route, to get a better vantage on them.