Update
I got my PIN today, finished the enrollment, and made the payments and scheduled future payments.
The system works, but I found it stupid and clunky, like something from the 1980s.
For example, you enroll, then get your enrollment number and your PIN in the mail. Then you go to the site and give your PIN and part of your enrollment number to get a chance to create a password.
Next, you can log in, and you use your SSN and your PIN and your password. IOW, you have two passwords. Two passwords are not more secure than one password, they're just more hassle.
To make 1040 ES payments, I have to select the "1040 tax form" not "1040 ES" which is what would make sense to me.
I went through the process of entering the four estimated tax payments and dates, and the confirmation screen shows "Not Required/2011" for the tax years, even for the last payment. The "Not required" was not helpful.
I then get to the final step, choose Make Payment, and get a "System error -- call this 800 number." I call the number, but am not willing to give my SSN in case I've been phished.
So I go through everything again, and this time it works. I'll see whether it deducts two payments each period.
So, bottom line is that this thing works, but it wasn't designed by tech-savvy people, and I don't really trust it. If I were doing it over again, I'd just pay electronically with my SSN and 1040ES on the checks.