Fredhopper

Pros: A lot of easy controls in their merchandising studio for everything from filters to ranking to result modifications to search settings, to presentation attributes to a lot of custom triggers that you might need on front end, while allowing to incorporate their AI modules of AI scores and AI search as per choice.
we are dealing with a wonderful supporting team with enough flexibility to offer you their time to find solutions together and not just dump that this is what it is and you deal with whatever you want to do on front end. they are happy to put smaller customizations in place for clients which are more configuration oriented on their side that makes it a worthy relationship.
They are evolving their integration mechanism to be completely API and streaming based that would be cutting edge in industry.
Their preview environment is good (could be made more useful with UI/uX improvements as well), also allows a prepublished and published API version from the same environment. they can offer n number of test and live environments as per our need.

Cons: Bit naive initially if you are integrating based on CSV files. localization of values is troublesome. handing of date attributes is out of question. every piece of consulting is charged, so need to use those wisely. there are some UI/UX lag in the tool that should be improved soon, specially on the preview tool.

Overall: Loved working with them and brainstorming together to find creating solutions to consumer problems. their openness to make it work with anything and everything is really good. So, I'd recommend them any time to other people who need the tool.