I rarely re-enter and rarely take the add-on, but that's just my cheapskate personality.
Re-entry is a profit-neutral decision as long as your starting stack size is significant. Re-entering and receiving a 10-big blind chip stack, well, you're not a strategist, you're a gambler (and that's fine, just realize what you are).
Add-ons, I view them a bit differently, being a cheapskate... I can almost NEVER justify doing an add-on except if the fee is very small compared to the tournament buy-in, and that's usually not the case. Usually the add-on is a significant fraction of a full entry fee and thus I pass on it. Buying extra chips guarantees exactly nothing, so I view it as paying something and getting nothing of real value, so I pass on it almost every time.