Date of use:1/2008 to 10/2011
I have been using a GMX e-mail account for about 3 years and had no major problems with it.
I got it to replace Hotmail when Microsoft got rid of their DAV access system (for e-mail program access), thereby eliminating all access to the junk mail folder, except through their website, which meant I was denied access to that folder as I will not stoop to using a web browser to access e-mail except in emergencies (they are not designed to handle e-mail, nor do they transfer that data to my computer like a real e-mail program does). I was using Hotmail, and now use GMX, as an e-mail address for any website that insists on my giving them an e-mail address for a purpose that I agree to.
The only websites that have had a problem with GMX has been MyPoints, which had problems getting their messages to me to go through, and www.eversave.com, which refuses to recognize the validity of my e-mail address, and won't respond to my messages to them regarding that problem. (They might have a problem recognizing GMX since it is UK based, not US based.)
GMX doesn't appear to have any employees, at least they don't have a responsive Customer Support Department. That can be a problem, but they have a help page and a web forum. (I'm guessing they are supposed to replace a real person.)
GMX is easy to set up in your e-mail program and has a nice feature: every day you get an e-mail, listing the new messages in your Junk Mail Folder, downloaded with your Inbox messages. This message lists the Sender (including the actual e-mail address, which you normally can't see without opening the message), subject, and a code indicating what method they used to determine that the message was spam/phish. The message includes a table to decode the codes. This allows you to know if something got sent to the Junk Mail Folder that shouldn't have, and why it got put there. (So you can break the rules to retrieve it, and giving you the option to mark it as safe.) Reviewing the Junk Mail Folder in your web browser is a bit of a pain due to the refreshes, but its livable if you keep the junk under 650 messages, or so.) The only time you don't get the message is when they have a problem, when they do they catch up when they resolve the issue. If Hotmail had this feature I would still be with them.
GMX does have another feature, that I'm not interested in, and haven't used, that allows you to retrieve all e-mail, from all your e-mail accounts, on their website, if you are interested in that.