In general, if the rebuy option is going to make you get loose, just because there is a second chance, this is not good. Re-entries are getting more popular nowadays, and they will remain in scene for a quite long time, because it gives recreational players this 2nd chance, and this is more money for the rooms, but also for good poker players. It is amazing if you identify a fish who just gives away his chips and keeps rebuying.
Both rebuys and re-entries are ok in this situation: imagine you are playing a great MicroMillions event that has a lot of recreational bad players, and you know that you have an edge on the general field. Also, it is a tournament that just happens once a year, and the guarantee is great. Imagine you get out with KK vs. AA, for example. If you have enough
bankroll, one rebuy or re-entry is good if it's not too late to register and enter with a short stack.
Also, let's say you play $5.50 tournaments regularly. You might try $1 tournaments with rebuys/add-on, but being disciplined to not rebuy if it is too late, for example. If you spend $1 to enter, $1 in a rebuy and $1 in add-on, you have spent $3, when your average buy-in is $5.50, which can never be bad.
About add-ons: if you are playing a tournament with add-ons, consider its price before entering the tournament. If it is a $1 tourney and add-on is also $1, you already have to consider that it is a $2 tournament, if you decide to don't rebuy. Almost everybody is going to make an add-on, and you should be making too, because you will get far behind if you don't make it. But if the add-on does not make much difference in stacks, you don't have to make it.
For example: in 888poker there is a $3.30 R&A. Initial stack is 1,500 and the add-on gives 3,000 more chips. This looks quite few chips. In this tournament we can expect few add-ons.