My CSV is quite large and is stopping at 21% after about 1 hour. Went to the product list and saw that exactly 99999 products were imported (?).
I got no errors, no logs, nothing, just the dreaded Ajax error page with the "proceed to the error page" link leading me to the import page with the grey/disabled "Importing - (21%)" button.
Tried as well with the alpha6-(dev) version.
The issue at http://drupal.org/node/1029102 seems unrelated to this one, as the file is already on the server. There it was suggested to do a "hack" to the database, but then it might stop again at 21% after all that time it takes to get there.