I have had trouble with the Constant Contact autoresponder series for two reasons:
The number of newsletters you can send in one series is limited to 15. That creates problems for us as we send out newsletters every two weeks forever. There should be somewat to either add to the limit or link the limited series to a new series so I don't have to add the contacts manually to the new list as they finish up the old list.
Sometimes we go longer than two weeks between newsletters due to busy schedules, travel etc. Basically when you break the autoresponder pattern, then those people who were supposed to receive a newsletter during one of the breaks fall off the list completely. They do not restart when a new newsletter is added. When this happens, I have to manually start a new list and move those contacts over to the new list. This is very frustrating and time consuming. Also after this happens, each new newsletter must be posted to multiple lists. There should be a workaround for this.