Retargeting can be effective, but it starts getting really annoying a few weeks after you decided against a product and for me creates a negative image of that company because I feel almost harassed. If you use it, I would recommend trying to limit the time frame or frequency of ads shown to something reasonable. Maybe a lot of impressions for a few days, then a few impressions per week for up to 2 weeks, then maybe just a few brief impressions after that. But don't go on for months.