I have done that. I created a csv file and uploaded roughly 5500 products directly into the database. I put the categories in manually then created another csv to upload the linkage between products and categories. This is where it fell apart and I have not resolved it yet. Although from admin mode, checking on the products I could see that they were in the proper categories but in user mode it would not find any products when you clicked on a category. It would just freeze for 5 minutes while searching the return nothing. Another issue you may have is that in search view the text size is fine but in detail view I find it too small. This has to be handled manually for each product. There is no uploadable database field that I could find that changes font size on the description and these are not controlled by a csv but rather within CMS.
So yes you can easily get all the products in but as far as I've been able to work out you will have to assign them to categories manually. I've gotten pretty good at importing though so if you get stuck I'd be happy to help you with what I know.
