Rebuy Personalization Engine

The latest version actually updated their discounting to use Shopify Functions which makes this app usable so I recently reinstalled it.

The good. It does a lot in one app. GWP, Tiered GWP, Cross sells, Bundles, Post Purchase.
My main complaint that this app is not intuitive and although their support is responsive it does take a lot of time to get this app up and running the way you want. There is a ton of time spent on troubleshooting. I am still trying to get GWP to work. I am currently using a different app (that literally took 3 min to set up) for that because I am still working on getting it to work in Rebuy.
Another time suck is their inconsistent CSS styling across their components and carts. This means you will have to spend extra time writing CSS (hate!) making this app appear consistent across your website. When asked about this their support wrote:
"The widgets and Smart Cart just happen to have different styling. For instance, the line items are bolded in Smart Cart but items in widgets are not. Please note that we cannot provide support for customizations beyond the scope of Rebuy."
So my take is that app comes with a lot of functionality/stats but expect to also spend a lot of time on getting it right.

We have had a great experience with this app and have seen our AOV increase from the start. Customer service is super kind and helpful and assisted us with a billing issue. Pricing is questionable though.

Edited: The app was sold to us as a $99/mo solution. It turns out it is a sliding scale depending on how much business you do with them. The app sometimes takes "credit" for making sales that it didn't impact so that is really frustrating.

Dante at Rebuy is the best customer support person I have ever had the pleasure of working with in my 14 year career. The Rebuy Engine is powerful and he goes above and beyond every time to make it easy to use. We especially like the smart cart feature on Rebuy.

-Update:
After posting this review the Rebuy Team got in touch and affected fixes in their app and got us up and running - which is great. However, it still took 4 months and a negative review to get a result. If only this happened sooner we'd be giving a much better review. 3 stars.
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Really disappointed with our Rebuy experience. After looking at many apps, we specifically chose this app because of its stated compatibility with Multicurrency. / Shopify markets AND the white glove setup at US$1000 - which we ended up doing ourselves anyway.
After literally months of back and forth the app is still not working with currencies, and while we have had our $1000 setup fee refunded - subsequent communications with Rebuy have stopped, they’re simply not replying.
Our last few emails have not been replied to, which is extremely poor. After many hrs of setting up rules were uninstalling and looking at alternatives

It works fine but then I found my abandoned checkout got higher. And many customers told me they were unable to checkout. Shopify support team found the cause is from Rebuy. So I had to remove the app. Nice functionality but it doesn’t mean anything if my conversion goes down

A major part of the reason for upgrading to Shopify Plus was for Rebuy. We were told that the app wouldn't slow down our site. After working with the Rebuy team (great people and very helpful which is why I'm giving 3 stars) to set up our widgets and data sources we set Rebuy live. Immediately our site speed plummeted (lighthouse score went from 85 to 30) which in turn sank our conversion rate and made our ads more expensive to run. We were told Rebuy runs after our pages load, but I'm pretty sure that's not true. After about 2 weeks we realized what was going on and uninstalled the app. After another week of scrambling we realized that our site was littered with rebuy code we decided to revert to an old theme out of desperation. After reading other reviews I've only found a few other companies that had this problem, so maybe it's unique to a few. That being said, although we feel we were misled, we have been in communication with the Rebuy dev team who said hey are working on optimizing the code even more. We will try Rebuy again after that happens, but we will be way way more cautious this time around and have a back-up theme in place just in case.

We tested the app because the features looked interesting. First I started testing the Smart Cart:
1. the preview does not work properly
2. it slows down our entire page load for more than 300ms
3. it decreases our mobile rating on Google Lighthouse by about 20 points
4. the theme I set up does not show up in the preview when adding a widget.
5. since the preview doesn't work, I tried adding it to a draft theme, which doesn't work either.
6. the styling options are only updated after saving and reopening the page - not really user-friendly. After experiencing that the preview mode doesn't work properly and the page load is slow, I decided not to test further. I would be happy if the above issues are fixed, then we can test further and adjust feedback. The basic idea is great, just need to improve the performance and UX. --Update--
I just spoke with John for almost 2 hours to fix the problems. We were able to verify the problems and he promised to work on it with his devs. I will keep this rating updated. For now, I am raising the rating from 2 to 5 stars because they got back to me and took the time to troubleshoot the issues. I will update it as the ticket is processed.
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pageload was not fixed, I waited a lot but no real help. For me this is absolutely not a 5 star app. It has cool features but the performance is poor. We had to uninstall the app.

I can see the potential of this tool. The offer of free support is nice, albeit hard to find within the Rebuy app (didn't see any link for support or help, despite the onboarding email stating "chat with us in app"). And I need help, because the learning curve is extremely steep for non-technical people. Building data sources separately from the widgets, settings like "Return product metafields" and "Default Endpoints" and "Set min and max breakpoints", and the use of boolean operators and logic trees will be appealing to developers and people used to working with code. For everyone else, it's confusing and a barrier to use. I watched all of the help videos, but I've still spent hours of trial and error trying to get a simple upsell widget to work the way I want. I also keep on getting kicked out of the app while I'm in the middle of something, which sometimes results in my changes being lost.