wpmudev is great at keeping the product current and working for the latest versions of WordPress. It's a really robust product that has a lot of things already included. Great for start ups with a few products.
What they're not good at is keeping up with the times. Marketpress 3.x was a major release which broke all of our sites when migrating from 2.x to 3.x. We needed to rebuild all the products, even with technical support. Since 3.x the community has been asking for reports, or at least the ability to export the sales. Nada. They'll fix and make minor enhancements but won't bring back features like reporting or exporting. They do have a strong community that will really try to help and their support is great, but this product isn't high on their radar. Great if you're a start up and don't need to do any reporting or have a lot of products.
If you're limited on money and budget, start here, but you'll need to swap it out to WooCommerce eventually, which is good but can get really expenses with yearly licenses on any enhancement.