Works, but has limitations most users won't face
I did run across a few limitations that a few users might face. First, if you upload a very large CSV file it attempts to load it into an array for validation and processing. The issue with this is that memory limits in PHP will prevent very large uploads from working. I hit the limit on my server around 50k users per file. In order to get around this limitation I used a CSV splitting utility and then uploaded and processed each.
Second, I believe because of the way the importer works (which is important so it doesn't bog down c5's queue), you have to remain logged in and with the importer on your screen while it works. In most situations this is fine, but when processing 50k users it will fail at some point and you'll need to manually restart the queue.
Ideally two improvements could be made: automatic splitting and handling of very large CSV files (mine was 250k records, which has taken around 4 days to import), and ability to work completely unattended without needing to be logged in.
As I said, though, for anyone with even a few thousand users to import, you'll find this add-on very easy to use and very efficient.
User Import - THANK YOU!
An excellent Add On
The import/mapping of user attributes is well thought out. The validation/live import sequence is really useful for the often complicated task of matching different field types.
Overall, both add-ons display nice, are easy to use and testing shows good upload of ALL the attributes you might have set up for users, both standard and custom. I also particularly like the ability to designate user groups at the upload time.