We're in the process of migrating from Zen Cart to Magento, and hoping to go live with the new site early in 2012. The shopping cart migration service was excellent. We were able to migrate about 1,000 products and about 2,500 customers within just a few hours (started migrating in the evening, and the migration was complete by the next morning). You should know that you can run the test migration multiple times to allow you to double-check your settings - this is a huge time saver. Also, after your migration has been completed, you can do incremental migrations that will only capture new data. We'll be using this feature as we get closer to going live. Overall, I give this service a 10 out of 10. It was super easy to set up, and worked exactly as advertised. We'll definitely be using it again and recommending it to others.
Scott Benton