Having been in the industry for 25 years my experiences with ZS have been both good and bad.
For the good: they do what you ask them to. They have a playbook and can do things consistently with few errors. They present well (mostly). You can use them as political cover for sensitive projects.
For the bad: Complete lack of transparency. Even though what they do is not innovative in any way, they turn it into a black box and won't reveal their 'intellectual property'. Guess what, it's a regression or some other standard stat technique. Don't play it like you've found the answer to nuclear fusion. Which leads to my next bad - anything that could be considered innovative happens because they patently steal ideas from their customers (even after having NDAs) and turn around and repackage them and sell them as their own. I have personally experienced this twice. If you use them for primary/secondary market research, beware, they'll miss obvious market events.
Overall if you do use them shadow what they do because you will likely come up with different/better results. If they do alignments for you and place reps, make sure that they rely on realistic mapping - don't place a rep who lives near Hartford, CT into a territory in eastern Long Island, NY, unless you are OK with them taking ferry every day or driving 4hrs to reach their territory. 'As the crow flies' doesn't cut it.
Bottom line, use them at your own risk.
