Not very good. A client can pay by check, but then you can't mark an invoice as paid!
The client login doesn't work well. When you create the client, it doesn't send them an email to create a password and you cant do it either. you have to leave to the user tab and do it from there BUT if you create the client in the user tab, it will not recognize the user as a client.
Also, permissions are bad. Only and admin can manage invoices, no levels.
Limited payment options. If you look in the comments they've said for years they'd do recurring payment system and they didn't