It initially looked kind of nice—no ads in the web interface, the ability to turn their newsletter off (which I already get about five times due to multiple accounts), text-based spam filtering (probably some kind of naive Bayesian stuff).
Then I started noticing that the text-based spam filter caught legitimate mail; before that, the only one that occasionally caught ham was the server-based one, which only accepts mail from certain large domains such as Hotmail when it comes from that domain's email server (a kind of mini-SPF scheme), which falls afoul of (a) forwarding and (b) From forging, both of which are not uncommon.
But today I noticed that GMX doesn't seem to adjust displayed times for time zones, which makes it difficult to compare arrival times of various messages if they originate in different time zones, and it doesn't use an email message's charset for display (so, for example, LiveJournal's UTF-8 comment notification mails look messed up if they contain non-ASCII characters).
FastMail.FM handles both of these things just fine. So Ack Ptui at GMX.
I mean, really—I can kind of understand if you don't want to change the charset parameter if you have a bunch of nav links in a frame around the message and you don't want German umlauts to look Cyrillic or Greek, but time zones? It's not exactly rocket science, mate!
I either expect a time zone indicator next to the time in the overview, or (preferably) to have the time automatically adjusted into my time zone. Or even have them all in GMT/PST/whatever; that's still better than displaying each time in its own time zone.
I'm also a bit annoyed that I can't seem to view the full headers of an email message, and that attachments often show up without names, giving no clue as to their identity.
The web interface sucketh.
Unfortunately, I've used the GMX address forever, pretty much (probably since around 1997), so it'll be difficult to migrate to another one such as one hosted at FastMail.FM, even if I prefer their interface. (And it'd even be cheaper—the Full account which I'm using now for my main FastMail.FM address works out to $1.66 a month if I pay yearly and $1.46 if I pay every two years; that's less than half of what GMX ProMail costs.)
GMX was my second ever public email address/forwarder, after a writeme.com one (then on iName(?), now part of Mail.com) which uses 'pne' and which I decided not to use as much since I imagined people can remember an address based on my name better than one based on a login.
Ah, le sigh.