I heavily played the original ZZT and Super ZZT the most. As they were both Shareware, I never did actually beat them but played the hell out of what the shareware versions did include. I always dreamt about making my own levels in the "for pay" version but never had the money to actually spring for it.
I think one of the reasons I love MUDS, ANSI adventure games like ZZT and the like and find more modern incarnations in games like Dwarf Fortress is because of the imagination you have to bring to the game. When the player is a simple smile on a screen the epicness of what you are doing lies in your mind. Another reason these games are awesome is because they are often simple to play.
One awesome trival-bit about ZZT is that it was made by Tim Sweeney and a very early Epic MegaGames. Yes, ZZT was made by THAT Tim Sweeney and THAT Epic Games... the folks that brought us Gears of War and Unreal Tournament. Kind of awesome to see how far the company came from making ANSI based adventure games to writing the tech that powers the majority of triple A games in the industry.
To many ZZT wasn't a huge footnote in gaming for them, but for me it was a pretty large DOS staple back in the day. If you could download it off a BBS in an hour or less I played it and ZZT was one of those games I played the hell out of.