I have had a generation 1 Airport Extreme since launch.
With ADSL it was a pain as there was no built-in modem, but as mentioned above, FTTC requires its own OpenReach modem, so I don't mind so much anymore.
The Airport Disk is fine and I can share it with all of my machines around the house (even Xbox 1 running XBMC). Spin up times on the HDDs are slow though, and it sometimes makes for a sucky experience (navigating folders when it's spinning up). This could obviously be my 3.5" HDDs though, a 2.5" one I had for a while was much quicker.
Time Machine backups cames along soon after the device was released, I think it was late 2007 or something, and they've worked brilliantly for me ever since. Never have had any problems syncing or retrieving files. I partitioned my current 2TB HDD into 750GB for Time Machine and the rest for user content. Therefore once it runs out of space backing up my 128GB and my other 256GB MacBooks it will start to get rid of old files within its own partition and then I know there will always be some space on the user partition.
I'm probably rambling now, but the only downside like others have said is the software.
They updated it last year sometime to version 6.0 and they removed a few features which I used. DHCP Client ID being one of them (used to set the BT Infinity username/password). I use v5.6.2 or something similar to configure this now. Pain in the ass.
Yeah, they're alright. I would buy again.