The out-of-box (OOB) features that come with the platform are fairly robust, and are more than enough to craete a professional, customized and feature-rich webstore. Template driven pages with timers and preview make it easy to customize the site and allow for changes at any hour. Built-in category structure and product groups are also very well engineered.
The discount engine has come a long way, and I really haven't seen anything that is quite as good. Managing discounts can be tricky, and ML has mastered it. They truly do try to be best-of-breed, and provide all the latest technologies and features. The caveat to this is that it may cost you. (See dislike section.)
Having ML host our sites as well as provide the software has proven to be an excellent decision. Very rarely is there any sort of hosting issue; and if there is, it is handled almost before we are aware of the problem. Seamless is the right word here.
Our biggest pet-peeve is that anything out of the ordinary, such as integration to a back-end system or an alternate pay method can be costly. Support tickets can also be costly, depending on the cause of the problem.
Overall, Marketlive (now Kibo) provides us with a stable, professional set of websites to conduct our e-commerce business, and has done for for 11 years. For the most part, it has helped us keep up with the ever-changing landscape that is the internet.