As far as we know, SSRI withdrawal does not cause permanent damage. But it feels like a bad flu, so it's not fun.
The withdrawal is based on two factors: the half-life of the drug and the "potency" of the drug. The short half-life drugs such as Paxil tend to produce worse withdrawal than longer half-life drugs, such as Prozac.
Many psychiatrists take advantage of the long half-life of Prozac and prescribe a single dose to treat withdrawal from the other SSRIs.
Importantly, there is cross-coverage from other drugs. Stopping Paxil cold will produce withdrawal, but switching Paxil to Zoloft, for example, should protect you from it.
In addition, this withdrawal effect is (mostly) limited to serotonergic antidepressants. Wellbutrin is not expected to have this effect.