In 2E I used Core Rules 2.0 and in 3E I've tried to use a few programs. Honestly, the most good I got from Core Rules 2.0 was the PDF books which were included, because I didn't have a print copy of Spells & Magic at the time. I had some fun mucking around but never got much more convenience out of it- then again, 2E was not as modular.
E-Tools is atrocious. Key elements are left uncustomizable so that you have to pay $30 for single book expansions which don't cover a quarter of what the actual books do. This alone is enough to make me slap myself for buying it, but the interface is also clunky. My last project was to give a dragon class levels, and I tried to do this from the dragon monster entry. You can't do that. Next I tried to enter through the character generator and choose a dragon race, but this couldn't be done either. Finally I found a slightly different character generator window which would allow me to combine the two, but I still had difficulty actually adding levels (Had to pass the invisible ECL first), assigning Cleric spells, keeping Dragon and Cleric skills seperate, and feats, don't get me started on the feats- Adroit Flyby is not in the Draconomicon data set, and feats listed in multiple books can be taken multiple times, but in the case of Improved Toughness the bonus is never added. Lightning breath gets an attack bonus. It's just not a very good program.
My experience is that electronic DM aids are not convenient at all except for the cartography programs, and those are full of problems too. Nowadays I just keep long rambling notes in Wordpad and it works just fine.