What does MagneticOne Store Manager do for me? Ease of administration, runs as local app and hits database directly. Can import well? But reindexing is done through Magento - still grinds.
What does MagMI do for me? Mass Import and more - but only compatible with Community Edition. Does a great job of importing massive numbers of SKU's and indexes them on-the-fly as well, but once those products are in you still have the issue of Magento grinding by simply looking up a product, etc.
Using MagMI I can upload one million products into a stock Magento CE installation, but once I'm finished I can't even seem to get a response to a simple search query.