IE 4 and 5 were innovative (IMO as someone who used both at the time - 1998-2000 - and actively converted family members to IE) compared to Netscape: it had a cache which worked consistently (important in 28.8 modem times) - Netscape would ignore the cache in some situations, i.e. resize the browser window and it would re-download images, even though it had them in its cache, and also IE had things like smooth scrolling which helped make things "nicer" to scroll and feel better from a UI perspective, and things like "make favourites available offline" feature, where it would download a bunch of full pages (whilst you were dialed up), and you could browse them after you disconnected.
After IE 6, things when downhill fast with the stagnation, but before that point, IE was a good browser.
The biggest issue with IE is it was HEAVILY integrated into windows. That in turn made it really slow to move. To get IE 6, you needed windows 2000, to get IE 7/8, you needed XP, to get 9+ you needed Vista.
That particularly became a problem because the time gap between XP and Vista was huge (and a lot of people skipped it and went to 7/8/10). In the meantime firefox and chrome came up and started innovating rapidly. Chrome started it with the evergreen model and FF quickly adopted that model.
After IE 6, things when downhill fast with the stagnation, but before that point, IE was a good browser.