TECO got easier to use with time, especially after the MIT people added ^R (Realtime) mode, which showed a whole screenful of text at a time as opposed to the earlier behavior where TECO would print on command but was otherwise like ed instead of vi. Makes sense, seeing as how TECO originally stood for Tape Editor and COrrector, as in paper tape.
TECO's main claim to fame was its command language, which became its programming language: People wrote programs (including the original EMACS) in TECO's command language, where most commands were non-printable characters in ASCII. There were conventions to print those characters readably (well, readable for human TECO users), such as printing Esc as $, but the fact remained you were programming with line noise. ;)
There's a TECO for modern systems, called Video TECO, here: