I have done 2 migrations with Cart2Cart. The first was bigcommerce to magento and the second was shopify to woocommerce.
The first migration was very complicated and included configurable products, so the service didn't end up working out well. I don't put that on cart2cart though, as bigcommerce and magento handled the products in very different ways. Even a developer writing that we hired later a custom script had issues making the import work.
Our second migration was a straightforward job from shopify to woocommerce. I made several mistakes in my selection, yet their suppor team worked with me to fix my mistakes in my original order and ensured the migration was completed correctly.
If you're doing a simple migration (i.e. no configurable products) for data fields that are supported by the shopping carts, then cart2cart is great. For more complex jobs (i.e. configurable products, lots of custom fields, etc), consult with support before making a purchase. I will use this service again.