Works great ! Initially had some problems with the install but once i figured it out its extremely easy and simple integration. Here are some tricks if you are having trouble:
1) The zendesk shopify app is just a link to zendesk. You have to go to your zendesk profile and go to apps and search for shopify to get the real integration.
2) Once there the app description will pretty much tell you what to do, like how to get the api keys from your shopify. This said, there were a few confusing points to take into consideration.
a) your shop url is in the form: https//:storename.myshopify.com . Use this format when installing the app
b) The order id filed id, is the unique id of the ticket field that you have created in zendesk that will have the order number in it. This can be found by going to the ticket field section, clicking on the new ticket field that you have created. The unique id will then be shown. Its usually like a 6 figure number.

Once I got these things settled all worked fine :) !

Amazing App, and great integration.
If you're having trouble getting it to work, make sure that your store address starts with https and not just http

Update:
I finally got reply from the manager, Maxime, after he reviewed my case, he helped me right away, and respond my email within a day.

and now my zendesk works perfectly.

I changed my rate from 1 star to 5 stars, all because Maxime's help.

If you are looking for customer services app on shopifty, Zendesk is the best choice, and if you have low budget, you always can start at as low as only $1.

you wont regret.

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Like the concept of Zendesk,
But i cant install the zendesk with my shopify and contacted with support team almost a month. But i did not hear from them for almost two weeks.
supper disappointed.
Do not waste your time to deal with them.

Zendesk is nice. It's easy to reply to Facebook, Twitter and emails on iPad, iPhone, android device and desktop. And especially cheap for first one year.

Zendesk is amazing. Great support. Use the voice feature also for calls.