Cross Sell & Upsell by SSC

It works well and they are listening to suggestions. Right now it's not really practical for really big collections but I've made a few suggestions and they've added them on their roadmap so it already deserves 5 stars.
It looks nice on the front end, with several options to match your theme.

Support is excellent, this app really does work well!

Awesome app with great team. I always get quick support from them. App cross sells my products very well.

Works great and simple to use :)

Easy install and super responsive support from the team who helped to get the styling pixel perfect. Nice one.

No BS when I say it's an easy to use and install app. It does the job VERY well and looks clean. Also, there's staff that are able to help you install or configure whatever you need, 24-7. Best app around.

Super easy to use, set up the cross-sell in minutes! The support team is very helpful and responsive, recommended!

Unable to attach promotional discounts to cross sell pop-up
Limited functionality and customization

Amazing support - Stuart was super helpful in getting my installation set up. Very easy to use.

Still too early to tell of it has made a difference to AOV but everything so far has bee great - highly recommend