so far I have tried every piece of open source carts : prestashop, magento, oscommerce, zen-cart, you name it I've tried them all (like most of the people on this forum). Also i tried many commercial ones just for fun : Interspire cart (Bigcommerce), CubeCart, Volusion, ... .
So whats the thing? I like Open Source projects no matter if it is an application, design, or something else. I use to be a programmer when younger (drop that completely ) and I spend hunderds of hours looking at the code of apache, sendmail etc. I participated alot too because I believed (and still do) in open source. I still believe it was well spent time. Im telling you all this because in further text I will have a critique on Open Cart not because I hate it, but because I really like the code which is pretty much well organized , not perfect but good. I mean, when you live trough some other BIG open source projects this one looks amazing.
Anyhow, Open Cart looks promising but that means nothing. First thing I do not like in Open Cart is that I have to pay for something that suppose to be included in a software. Example of that is "Product comparing" , rotators etc. I know, some programmers who made those modules will jump on this one, but open source is about progress and everything that comes with it. It all comes down to this : if you want to make money then you should make modules for commercial carts. Period.
What I like about Open Cart is that I actually see what features will be included in next release. This clearly proves me that someone actually have a real roadmap. Good work Open Cart.
Second thing is templates. There are many types of software like : fast software, slow, simplistic, complicated, nice looking, ugly, .... . first impression is the key if we have a software that has to deal and to be seen by many. Cart software is like a girl. You allways pick the one that you personally like. You never pick a girl you don't like thinking : I will get her few implants there, make her do lips, front and behind, ... lol you simply choose a girl that has all that and live with it. Same thing is with shopping carts. Make outstanding default template that 90% will like and never look back. The best cart software (by many) is Interspire cart. Take a look :

Looks nice right. Dont get me wrong, default Open Cart is decent but unfortunately not enough. It looks better than Presta or Magento, thats for sure but if you want something incredible you have to do much better. There is a simple rule of success : if you want to be the best you have to beat the best. The look of shopping cart is as important as the code.
Other things are features. Its not about the amount of it but about quality and usability. Some features are a must. For example : get one product and have another for free (or have a discount on it). If you dont know exactly what features your software must have all you have to do is open your competitions page (commercial competitions page like Interspire) and copy the idea (with improvements). And that's the beauty of open source. Commercial companies simply do not have resources (ie. money) and time to spend it on features that may or may not be needed. All of their modules are made to make money so their own web page is a great resource of what one cart must have.
Also, one page checkout is great option and it makes money. I do not see that feature in Open Cart. Searching is another option because people actually have to find a product before a purchase. Advanced search must come with few features not only one or two like in Open Cart but with options that include : brand name, price range, if it is featured, with free shipping, if it is downloadable etc. Open Cart doesnt have that either. Also search tips near advanced search button is a nice addition. Having separate page explaining how to actually find exactly what you are looking for is always a good idea.
Also, some things could be implemented as default. In open cart, in upper right corner you have : Special offers, Bookmark, Contact and Sitemap. There is exactly 1 that should be there : and the winner is Contact. Bookmark is sitting there with no purpose and sitemap is something 99% people never click on. Special offer page must be more agressive not "hidden". Sure it is visible but in that section a customer looks for a Help not special offers. What I want as a customer in that area is to have links to something useful like : Order status, gift and other certificates (coupons in general), About us, return policy, shipping policy, reliability section (wich will assure my customer that buying on my shop is safe like walking), Help, Faq, ... . Naturally Open Cart has that but located in separate block which is not a good idea because those things should be visible right away and not taking much valuable space. I have a proof of that. If you ask average user where he/she expect to have Help option in a application he/she will say : up corner to the right. It is that simple. Help is there as all should all necessary info. In Open Cart you actually have to scroll down and look to the left.
From what I saw, Open Cart is fast, have clean code, but lacks in design and few crucial features. So far, the best OS cart on the market. Some will say : what about Magento? What about it? It is only matter of time when their team will have to rewrite the whole code and Im not even sure thats possible. In Magento, if you want to do something right you have to hire someone who really knows the platform. Looking around and testing is more or less interesting but for serious shop you have to spend lot of money and time. If I have to choose, I would rather buy Interspire or Volusion than to install Magento.
Bottom line,
Open Cart suffers from the same problems as other OS carts (some links are displaced or missing, or total luck of certain features) but I personally believe it is only temporary. If Open cart team decide to implement and improve existing features they would be good to go and even more. So far Open cart has serious advantage comparing to other OS carts and most of the commercial ones. All they have to do is to beat the best.
As a customer I want from a cart software to : load fast (Open Cart is fast enough), looking professional (very important), have everything on a silver platter, fast search, fast checkout (sometimes I don't even want to log in to my account) and a nice customer service at the end of the line.