I've only been thinking about this for a few minutes here so think of this as a first impression more than anything but;
As a small shop owner that barely broke triple digits in payouts over 12 months I can't see how reviews would be anything more than yet another obstacle for smaller shops to overcome. This is still the case even if ratings don't influence things like search/marketplace placement. It's just a numbers game, a single bad review will have a huge impact on an item if that's the only review it has, sure it might balance out once you get a statistically significant of reviews but that's less likely to happen if you don't get a bunch of positive reviews first because bad reviews erode customer confidence, whether it's warranted or not.
To touch on the idea of Reviews somehow affecting how an item shows up for searches or the marketplace is something I don't want to see. From my perspective it's a "the rich get richer" type of scenario because if someone is getting enough reviews on a product to the point that it's "highly-rated" chances are they don't need as much help with marketing as someone smaller who is struggling. Now if/when you guys fix/replace the search I wouldn't be opposed to there being a way to SORT results by rating but I wouldn't want the results changed by rating behind the scenes as it were.
There is also the quirk of simply "updating" or re-uploading a model to clear out reviews, because it wouldn't make sense to have those persistent when the First to Try Success rate isn't.
Overall I think part of the problem is that as designers we have(at the moment) almost no oversight as to what you guys ship out besides the initial design. I mean, to be blunt, you guys aren't willing/capable of telling designers WHY/HOW a print failed and dropped their success rate yet having Reviews would, potentially, put a notable amount of power in the hands of customers who might not have any understanding of the process. In my eyes it'd require you guys to actively investigate negative reviews to determine where the fault stems from, was it an issue with the design simply not being suitable, something on Shapeway's end(packaging, QC ect ect) or is the customer simply wrong?
To be honest, given teh current state of things I don't think you guys are ready to take on that kind of responsibility yet. There are so many other things that will help all designers like customizable shop layout, functional search, ironing out the First To Try system ect ect), some designers(orientation, detailed feedback on failed prints prior to outright rejection) compared to reviews that would mostly help items that already see a lot of traffic/sales.
Off the top of my head I'd rather have something concrete, like showing the number of times something has been printed successfully. That doesn't rely on hoping that customers come back to leave a review that could easily be misinformed or just flat out false(regardless if there was malicious intent or not).