First off, Infopia is a very professional, customer-oriented company. If you want it, they'll build it for you. Don't get me wrong, though - the standard Marketplace Manager account is packed with powerful selling features.
The tools they provide make it very easy to sell on multiple channels. We use them to sell successfully on eBay and Overstock. In addition, the Marketplace Manager storefront website drives many add-on and upsells to our marketplace transactions. The MM Storefront also drives significant sales directly from the web, both from comparison shopping engines and search engines.
We've been very successful using their Configurator product to sell customized products on eBay (and our standalone website storefront). It can also be an effective cross-selling and package-building tool for certain items.
The best thing about Infopia is, that while they have very cool technology, they aren't just building pretty toys. They're building tools that are designed to help increase revenue and profit, in measurable ways. Infopia has been a major factor in the growth of our business, on marketplaces and beyond.